Fix get_bob_tags to return default tags

When passing None as estimator to get_bob_tags, returns the default tags.

bob/pipelines/wrappers.py

@@ -149,7 +149,7 @@ def get_bob_tags(estimator=None, force_tags=None):
         "bob_features_load_fn": bob.io.base.load,
         "bob_fit_supports_dask_array": False,
     }
-    estimator_tags = estimator._get_tags()
+    estimator_tags = estimator._get_tags() if estimator is not None else {}
     return {**default_tags, **estimator_tags, **force_tags}