adapt inlnies behavior

Author: bckohan

src/polymorphic/admin/inlines.py

@@ -53,14 +53,7 @@ class PolymorphicInlineModelAdmin(InlineModelAdmin):
 
     #: Inlines for all model sub types that can be displayed in this inline.
     #: Each row is a :class:`PolymorphicInlineModelAdmin.Child`
-    child_inlines = None
-
-    #: The models that should be excluded from the auto-discovered leaf
-    #: model sub types that can be displayed in this inline. This can be
-    #: a list of models or a single model. It's useful to exclude
-    #: non-abstract base models (abstract models are always excluded)
-    #: when they don't have defined any child models.
-    exclude_children = None
+    child_inlines = ()
 
     def __init__(self, parent_model, admin_site):
         super().__init__(parent_model, admin_site)
@@ -91,9 +84,7 @@ def get_child_inlines(self):
         This should return a list of tuples, exactly like
         :attr:`child_inlines` is.
         """
-        if self.child_inlines is not None:
-            return self.child_inlines
-        return ()
+        return self.child_inlines or ()
 
     def get_child_inline_instances(self):
         """