I don't see any way to make git add
's convenience globbing ignore a pattern that doesn't match anything, but there are much more powerful ways to build command lines.
First, a handy alias:
git config --global alias.files 'ls-files -oc --exclude-standard'
then
git config alias.acs '!git files -z *.cs *.csproj *.sln | xargs --null git add --'
gets you what you want.