Revert "Remove unnecessary guard"

This reverts commit 8ac0bb5.

The admin site somehow adds a RequestContext inside the list; a
RequestContext is no ``dict`` and does not have a .items() method.

Author: matthiask

debug_toolbar/panels/templates/panel.py

@@ -86,7 +86,7 @@ def _store_template_info(self, sender, **kwargs):
 
         context_list = []
         for context_layer in context.dicts:
-            if context_layer:
+            if hasattr(context_layer, "items") and context_layer:
                 # Check if the layer is in the cache.
                 key_values = sorted(context_layer.items())
                 pformatted = None