Sequencer: Fix python error on Add menu on default workspace

It is possible that context does have selected_sequences but
it will be set to None. In this case getattr() will return
None, breaking the intended logic.

release/scripts/startup/bl_ui/space_sequencer.py

@@ -36,7 +36,10 @@ def act_strip(context):
 
 
 def sel_sequences(context):
-    return len(getattr(context, "selected_sequences", ()))
+    selected_sequences = getattr(context, "selected_sequences", None)
+    if not selected_sequences:
+        return 0
+    return len(selected_sequences)
 
 
 def draw_color_balance(layout, color_balance):