DebugInfo: don't emit zero-length names for parameters

We check this in many/all other cases, just missed this one it seems.
Perhaps it'd be worth unifying this so we never emit zero-length
DW_AT_names.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188649 91177308-0d34-0410-b5e6-96231b3b80d8