[VFS] Handle empty entries in directory traversal

The VFS YAML files contain empty directory entries to describe that it's
returning from a subdirectory before describing new files in the parent.
In the future, we should properly sort and write YAML files avoiding
such empty dirs and mitigate the extra recurson cost. However, since
this is used by previous existing YAMLs, make the traversal work in
their presence.

rdar://problem/24499339

llvm-svn: 264970