VS: print comment in CSharp target only if it is actually set

16 jobs for vs_csharp_add_custom_command_comment
Status Job ID Name Coverage
  External
passed cdash

passed cmake-dash3win7-windows-debug-vs2015

passed ghostflow-branch-check-master

passed ghostflow-commit-check

passed ghostflow-stager

passed cmake-dash3win7-windows-debug-vs2015

00:44:39

passed cmake-dash3win7-windows-debug-vs2015

passed cmake-dash3win7-windows-debug-vs2015

passed cmake-dash3win7-windows-debug-vs2015

passed cmake-dash3win7-windows-debug-vs2015

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager