Fix getUserEmail
Fix getUserEmail

There is a chance that the Mailer property for a particular user will be
null when a user manually starts a job. In that case, return
userId@github.login.

dotnet/dotnet-ci,tannergooding/dotnet-ci,mmitche/dotnet-ci,mmitche/dotnet-ci,dotnet/dotnet-ci,tannergooding/dotnet-ci