Fix MemoryFileSystem to treat an empty path as non-existent (#213)

* Fix MemoryFileSystem to treat an empty path as non-existent

`_MemoryFileSystem.findNode` returns `reference?.directory` if given
the root path or an empty path.  However, an empty path should
be treated as non-existent, not as a directory.  Instead explicitly
return `null` as a special case when given an empty path.

Partially addresses https://github.com/google/file.dart/issues/198.

* Make _MemoryFileSystem._current nullable