glocalvfs: Create a dummy file for g_file_new_for_path("")

`""` is not a valid path (`stat()` on it returns `ENOENT`). Previously,
a full `GLocalFile` was being created, which ended up resolving to
`$CWD`, through path canonicalisation. That isn’t right.

Fix it by creating a `GDummyFile` instead, and adding a unit test.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2328

gio/glocalvfs.c

@@ -80,7 +80,10 @@ static GFile *
 g_local_vfs_get_file_for_path (GVfs       *vfs,
                                const char *path)
 {
-  return _g_local_file_new (path);
+  if (*path == '\0')
+    return _g_dummy_file_new (path);
+  else
+    return _g_local_file_new (path);
 }
 
 static GFile *