Beam: allow sending messages without attachments.
Beam: allow sending messages without attachments.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

mavenugo/swarm,doronp/swarm,rillig/docker.github.io,polyverse-security/swarm,cpuguy83/swarm,shakamunyi/swarm,bdwill/docker.github.io,atlassian/swarm,qianzhangxa/swarm,denverdino/swarm-1,u2mejc/swarm,SivagnanamCiena/swarm,yp-engineering/swarm,fugr/swarm,docker/docker.github.io,denverdino/swarm-1,thaJeztah/docker.github.io,menglingwei/denverdino.github.io,shot/swarm,auzias/swarm,deis/swarm,linecheng/swarm,mountkin/swarm,thaJeztah/docker.github.io,sanscontext/docker.github.io,BSWANG/swarm,BSWANG/denverdino.github.io,shubheksha/docker.github.io,auzias/swarm,joaofnfernandes/docker.github.io,rajdeepd/swarm,bdwill/docker.github.io,menglingwei/denverdino.github.io,sanscontext/docker.github.io,shin-/docker.github.io,denverdino/swarm,johnstep/docker.github.io,morpheyesh/swarm,jzwlqx/denverdino.github.io,denverdino/docker.github.io,vieux/swarm,therealbill/swarm,aidora/pisces,joeuo/docker.github.io,moxiegirl/swarm,jzwlqx/denverdino.github.io,sanscontext/docker.github.io,polyverse-security/swarm,dims/swarm,aduermael/docker.github.io,tnachen/swarm,menglingwei/denverdino.github.io,gdevillele/docker.github.io,jdamick/swarm,JimGalasyn/docker.github.io,alexisbellido/docker.github.io,xFragger/swarm,kunalkushwaha/swarm,ehazlett/swarm,affo/swarm,ezrasilvera/swarm,alexisbellido/docker.github.io,fugr/swarm,JimGalasyn/docker.github.io,menglingwei/denverdino.github.io,zkcrescent/swarm,docker/docker.github.io,beni55/swarm-1,joeuo/docker.github.io,chanwit/swarm,affo/swarm,LuisBosquez/docker.github.io,shot/swarm,PeoplePerHour/libswarm,errordeveloper/swarm,sanscontext/docker.github.io,liubin/swarm,ealang/swarm,francisbouvier/swarm,akerbis/armhf-swarm,pdevine/swarm,BSWANG/denverdino.github.io,bfirsh/swarm,pssahi/swarm,phiroict/docker,rchicoli/swarm,JimGalasyn/docker.github.io,megamsys/swarm,srikalyan/swarm,mrjana/swarm,jdamick/swarm,PeoplePerHour/libswarm,phiroict/docker,docker/docker.github.io,danix800/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,beni55/swarm-1,tranceitionalMynd/swarm,bpradipt/swarm,JimGalasyn/docker.github.io,joaofnfernandes/docker.github.io,therealbill/swarm,allencloud/swarm,denverdino/denverdino.github.io,kunalkushwaha/swarm,anweiss/docker.github.io,docker/docker.github.io,tranceitionalMynd/swarm,gonkulator/swarm,rancher/swarm,sanscontext/docker.github.io,ealang/swarm,srikalyan/swarm,schmunk42/swarm,bdwill/docker.github.io,LukasBacigal/swarm,mountkin/swarm,NunoEdgarGub1/swarm,joaofnfernandes/docker.github.io,linecheng/swarm,MHBauer/swarm,raoofm/swarm,phiroict/docker,ejholmes/swarm,jay-lau/swarm,JimGalasyn/docker.github.io,ricca742/swarm,rancher/swarm,alexisbellido/docker.github.io,FreedomWangLi/swarm,parapsyche/swarm,mrjana/swarm,jimmycmh/swarm,joeuo/docker.github.io,snrism/swarm,LukasBacigal/swarm,FollowMyDev/swarm,anweiss/docker.github.io,shakamunyi/swarm,londoncalling/docker.github.io,denverdino/docker.github.io,londoncalling/docker.github.io,tangkun75/swarm,swarm-hooks/swarm,adouang/swarm,rav121/swarm,aduermael/docker.github.io,dims/swarm,ehazlett/swarm,BSWANG/swarm,raoofm/swarm,troy0820/docker.github.io,shin-/docker.github.io,docker-zh/docker.github.io,qianzhangxa/swarm,yp-engineering/swarm,denverdino/denverdino.github.io,shubheksha/docker.github.io,denverdino/denverdino.github.io,mgoelzer/swarm,FollowMyDev/swarm,dnephin/swarm,aluzzardi/swarm,BSWANG/denverdino.github.io,londoncalling/docker.github.io,vlajos/swarm,docker-zh/docker.github.io,resouer/swarm,vlajos/swarm,denverdino/denverdino.github.io,iaintshine/swarm,tangkun75/swarm,CodeNow/swarm,tnachen/swarm,sfsmithcha/swarm,ch3lo/swarm,ejholmes/swarm,shubheksha/docker.github.io,denverdino/docker.github.io,phiroict/docker,marsmensch/swarm,rav121/swarm,unodba/swarm,adouang/swarm,Aorjoa/swarm,francisbouvier/swarm,sfsmithcha/swarm,bsmr-docker/swarm,CodeNow/swarm,LuisBosquez/docker.github.io,EderRoger/swarm,docker-zh/docker.github.io,abronan/swarm,zkcrescent/swarm,kyhavlov/swarm,mgoelzer/swarm,doronp/swarm,gdevillele/docker.github.io,kyhavlov/swarm,gradywang/swarm,anweiss/docker.github.io,shin-/docker.github.io,rchicoli/swarm,BSWANG/denverdino.github.io,johnstep/docker.github.io,echupriyanov/swarm,rillig/docker.github.io,abronan/swarm,Aorjoa/swarm,shin-/docker.github.io,parapsyche/swarm,ezrasilvera/SwarmACLs,gdevillele/docker.github.io,barais/swarm,everett-toews/swarm,rajdeepd/swarm,danix800/docker.github.io,jimenez/swarm,riuvshin/swarm,mavenugo/swarm,CiaranCostello/swarm,johnstep/docker.github.io,bdwill/docker.github.io,barais/swarm,xFragger/swarm,echupriyanov/swarm,EderRoger/swarm,phiroict/docker,iaintshine/swarm,rillig/docker.github.io,danix800/docker.github.io,docker-zh/docker.github.io,bfirsh/swarm,shin-/docker.github.io,jimmyxian/swarm,jeffjen/swarm,ezrasilvera/swarm,vieux/swarm,shahankhatch/swarm,Spritekin/swarm,danix800/docker.github.io,jzwlqx/denverdino.github.io,aduermael/docker.github.io,errordeveloper/swarm,gdevillele/docker.github.io,thaJeztah/docker.github.io,marsmensch/swarm,jay-lau/swarm,shubheksha/docker.github.io,Spritekin/swarm,johnstep/docker.github.io,FreedomWangLi/swarm,denverdino/swarm,pssahi/swarm,megamsys/swarm,cpuguy83/swarm,krystism/swarm,joeuo/docker.github.io,jimmyxian/swarm,jzwlqx/denverdino.github.io,moxiegirl/swarm,christophelec/swarm,denverdino/docker.github.io,londoncalling/docker.github.io,thaJeztah/docker.github.io,allencloud/swarm,MHBauer/swarm,morpheyesh/swarm,cgvarela/swarm,thaJeztah/docker.github.io,anweiss/docker.github.io,pdevine/swarm,krystism/swarm,u2mejc/swarm,ch3lo/swarm,johnstep/docker.github.io,rutulpatel/swarm,everett-toews/swarm,shahankhatch/swarm,cgvarela/swarm,christophelec/swarm,gdevillele/docker.github.io,denverdino/denverdino.github.io,BSWANG/denverdino.github.io,aluzzardi/swarm,troy0820/docker.github.io,ricca742/swarm,liubin/swarm,rutulpatel/swarm,SivagnanamCiena/swarm,jimmycmh/swarm,joaofnfernandes/docker.github.io,dnephin/swarm,deis/swarm,swarm-hooks/swarm,jzwlqx/denverdino.github.io,troy0820/docker.github.io,rillig/docker.github.io,aidora/pisces,unodba/swarm,gradywang/swarm,joeuo/docker.github.io,docker-zh/docker.github.io,NunoEdgarGub1/swarm,joaofnfernandes/docker.github.io,menglingwei/denverdino.github.io,bsmr-docker/swarm,gonkulator/swarm,schmunk42/swarm,bdwill/docker.github.io,akerbis/armhf-swarm,jeffjen/swarm,shubheksha/docker.github.io,CiaranCostello/swarm,atlassian/swarm,denverdino/docker.github.io,LuisBosquez/docker.github.io,alexisbellido/docker.github.io,jimenez/swarm,riuvshin/swarm,snrism/swarm,bpradipt/swarm,resouer/swarm,LuisBosquez/docker.github.io,anweiss/docker.github.io,alexisbellido/docker.github.io,chanwit/swarm,LuisBosquez/docker.github.io,aduermael/docker.github.io,ezrasilvera/SwarmACLs,troy0820/docker.github.io