tools/manifestfile.py: Fix freeze() when script is an empty iterable.

The documentation for `freeze()` says that:
- If `script` is `None`, all files in `path` will be frozen.
- If `script` is an iterable then `freeze()` is called on all items of the
  iterable.

This commit makes sure this behaviour is followed when an empty tuple/list
is passed in for `script` (previously an empty tuple/list froze all files).

Fixes issue micropython#14125.

Signed-off-by: Damien George <damien@micropython.org>

tools/manifestfile.py

@@ -291,7 +291,7 @@ def _add_file(self, full_path, target_path, kind=KIND_AUTO, opt=None):
     def _search(self, base_path, package_path, files, exts, kind, opt=None, strict=False):
         base_path = self._resolve_path(base_path)
 
-        if files:
+        if files is not None:
             # Use explicit list of files (relative to package_path).
             for file in files:
                 if package_path: