[Support][MemBuffer] Prevent UB on empty StringRefs

Empty StringRefs are usually identified by their length being zero, and
sometimes they'll have Data==nullptr (e.g. default constructed, or derived from
an operation like split/copy and result turned out to be empty).

If such StringRef objects are passed to llvm::MemoryBuffer::getMemBufferCopy,
it'll result in UB as neither src nor dst can be null, even if size is zero.

This patch prevents that UB by not issuing a copy whenever StringRef is empty.

Differential Revision: https://reviews.llvm.org/D144706