[lldb/Support] Treat empty FileSpec as an invalid file.

LLDB relies on empty FileSpecs being invalid files, for example, they
don't exists. Currently this assumption does not always hold during
reproducer replay, because we pass the result of GetPath to the VFS.
This is an empty string, which the VFS converts to an absolute directory
by prepending the current working directory, before looking it up in the
YAML mapping. This means that an empty FileSpec will exist when the
current working directory does. This breaks at least one test
(TestAddDsymCommand.py) when ran from replay.

This patch special cases empty FileSpecs and returns a sensible result
before calling GetPath and forwarding the call.

Differential revision: https://reviews.llvm.org/D77351