test: fs_helper: Skip empty srcdir when creating a filesystem

FsHelper.mk_fs() always creates a srcdir via setup(), then passes it
to the module-level mk_fs(). This fails for filesystem types like ext2
that do not support the -d flag, raising ValueError even when no files
need to be copied.

Pass None for src_dir when the srcdir is empty, so that creating an
empty filesystem works for all supported types.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/py/tests/fs_helper.py

@@ -66,8 +66,9 @@ class FsHelper:
         """Make a new filesystem and copy in the files"""
         self.setup()
         self._do_cleanup = True
+        src_dir = self.srcdir if os.listdir(self.srcdir) else None
         self.fs_img = mk_fs(self.config, self.fs_type, self.size_mb << 20,
-                            self.prefix, self.srcdir, quiet=self.quiet)
+                            self.prefix, src_dir, quiet=self.quiet)
 
     def setup(self):
         """Set up the srcdir ready to receive files"""