StringRef::copy shouldn't allocate anything for length 0 strings.

The BumpPtrAllocator currently doesn't handle zero length allocations well.
The discussion for how to fix that is ongoing.  However, there's no need
for StringRef::copy to actually allocate anything here anyway, so just
return StringRef() when we get a zero length copy.

Reviewed by David Blaikie

llvm-svn: 264201