unicode/utf8: don't imply that the empty string is incorrect UTF-8
unicode/utf8: don't imply that the empty string is incorrect UTF-8

Change-Id: Idd9523949ee4f2f304b12be39f8940ba34a420be
Reviewed-on: https://go-review.googlesource.com/16361
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

Cofyc/go,mwhudson/go,cyberroadie/go,deft-code/go,ganboing/go-esx,Endika/go,golang/go,momchil-velikov/go,golang/go,fjballest/golang,sndnvaps/go,polyverse-security/go,golang/go,akutz/go,Jordanzuo/go,miolini/go,4ad/go,roth1002/go,roth1002/go,linux-on-ibm-z/go,vsekhar/elastic-go,vsekhar/elastic-go,suncycheng/go,sndnvaps/go,CAFxX/go,akutz/go,deft-code/go,codestation/go,odeke-em/go,roth1002/go,vsekhar/elastic-go,roth1002/go,codestation/go,codestation/go,mwhudson/go,Samurais/go,vsekhar/elastic-go,akutz/go,binarycrusader/go,miolini/go,suncycheng/go,suncycheng/go,josharian/go,suncycheng/go,golang/go,nawawi/go,sndnvaps/go,theif519/go,christopher-henderson/Go,josharian/go,codestation/go,deft-code/go,binarycrusader/go,goodlang/good,fjballest/golang,Endika/go,linux-on-ibm-z/go,binarycrusader/go,odeke-em/go,polyverse-security/go,4ad/go,goodlang/good,Endika/go,codestation/go,miolini/go,fjballest/golang,cyberroadie/go,kim-racktop/go,odeke-em/go,cyberroadie/go,fjballest/golang,Samurais/go,fjballest/golang,ImJasonH/go,CAFxX/go,goodlang/good,binarycrusader/go,polyverse-security/go,Endika/go,cyberroadie/go,momchil-velikov/go,derekmarcotte/go,nawawi/go,fjballest/golang,nawawi/go,cloudflare/go,josharian/go,suncycheng/go,theif519/go,roth1002/go,goodlang/good,vsekhar/elastic-go,Jordanzuo/go,Cofyc/go,kim-racktop/go,Endika/go,theif519/go,josharian/go,Cofyc/go,sndnvaps/go,odeke-em/go,ganboing/go-esx,vanloswang/go,binarycrusader/go,Samurais/go,codestation/go,ImJasonH/go,linux-on-ibm-z/go,ImJasonH/go,roth1002/go,momchil-velikov/go,CAFxX/go,polyverse-security/go,Jordanzuo/go,Samurais/go,golang/go,nawawi/go,momchil-velikov/go,kim-racktop/go,josharian/go,sndnvaps/go,vanloswang/go,Cofyc/go,mwhudson/go,polyverse-security/go,Jordanzuo/go,linux-on-ibm-z/go,ganboing/go-esx,Jordanzuo/go,vsekhar/elastic-go,linux-on-ibm-z/go,miolini/go,Jordanzuo/go,vanloswang/go,deft-code/go,cyberroadie/go,theif519/go,momchil-velikov/go,4ad/go,deft-code/go,CAFxX/go,ImJasonH/go,4ad/go,miolini/go,Jordanzuo/go,binarycrusader/go,4ad/go,odeke-em/go,cyberroadie/go,4ad/go,ganboing/go-esx,ganboing/go-esx,Samurais/go,theif519/go,momchil-velikov/go,ImJasonH/go,fjballest/golang,Cofyc/go,vanloswang/go,nawawi/go,nawawi/go,sndnvaps/go,CAFxX/go,4ad/go,miolini/go,vanloswang/go,josharian/go,theif519/go,suncycheng/go,suncycheng/go,suncycheng/go,ganboing/go-esx,golang/go,mwhudson/go,polyverse-security/go,akutz/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,theif519/go,binarycrusader/go,linux-on-ibm-z/go,cyberroadie/go,goodlang/good,deft-code/go,Endika/go,deft-code/go,mwhudson/go,golang/go,derekmarcotte/go,miolini/go,kim-racktop/go,cloudflare/go,derekmarcotte/go,CAFxX/go,vsekhar/elastic-go,odeke-em/go,akutz/go,roth1002/go,odeke-em/go,roth1002/go,vanloswang/go,nawawi/go,Jordanzuo/go,goodlang/good,josharian/go,sndnvaps/go,polyverse-security/go,cloudflare/go,cloudflare/go,Cofyc/go,ganboing/go-esx,Samurais/go,derekmarcotte/go,derekmarcotte/go,mwhudson/go,Cofyc/go,linux-on-ibm-z/go,codestation/go,cloudflare/go,derekmarcotte/go,kim-racktop/go,mwhudson/go,cyberroadie/go,ImJasonH/go,CAFxX/go,momchil-velikov/go,ganboing/go-esx,theif519/go,odeke-em/go,goodlang/good,momchil-velikov/go,miolini/go,golang/go,goodlang/good,akutz/go,akutz/go,kim-racktop/go,sndnvaps/go,ImJasonH/go,Samurais/go,josharian/go,Endika/go,codestation/go,Cofyc/go,polyverse-security/go,4ad/go,mwhudson/go,vanloswang/go,deft-code/go,Samurais/go,Endika/go,kim-racktop/go,ImJasonH/go,kim-racktop/go,vanloswang/go,linux-on-ibm-z/go,binarycrusader/go,derekmarcotte/go,vsekhar/elastic-go,cloudflare/go,derekmarcotte/go,fjballest/golang,akutz/go