Don't add the 'Value' string if there isn't one.

This was causing the folding set to fail to fold attributes, because it was
being calculated in one spot without an empty values string but here with an
empty values string.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176301 91177308-0d34-0410-b5e6-96231b3b80d8