[new_pmlib] fix test source to work with empty msgs
[new_pmlib] fix test source to work with empty msgs

Glyfina-Fernando/pymtl,12yujim/pymtl,jjffryan/pymtl,jjffryan/pymtl,jck/pymtl,cornell-brg/pymtl,Glyfina-Fernando/pymtl,cfelton/pymtl,12yujim/pymtl,cornell-brg/pymtl,jjffryan/pymtl,tj93/pymtl,Glyfina-Fernando/pymtl,cfelton/pymtl,cfelton/pymtl,tj93/pymtl,jck/pymtl,jck/pymtl,tj93/pymtl,12yujim/pymtl,cornell-brg/pymtl