Fix AttributeError in base.html when called without response_headers

This happens when using a `404.html` which extends DRF's `base.html` and is called by a `Resolver404`.
Since `Resolver404` goes via django's `handler404` and not via DRF's exception handling,
it doesn't get the extra context.

Since the `|items` filter doesn't check for `response_headers` being null (missing) it attempts
to call `<None>.items()`, thus raising an AttributeError.

Originally I solved this in the template by guarding the `for` loop with a `if response_headers`.
However @rpkilby expressed a preference for doing the check inside the `items` filter instead.

rest_framework/templatetags/rest_framework.py

@@ -240,6 +240,10 @@ def items(value):
     lookup.  See issue #4931
     Also see: https://stackoverflow.com/questions/15416662/django-template-loop-over-dictionary-items-with-items-as-key
     """
+    if value is None:
+        # `{% for k, v in value.items %}` doesn't raise when value is None or
+        # not in the context, so neither should `{% for k, v in value|items %}`
+        return []
     return value.items()
 
 

tests/test_templates.py

@@ -0,0 +1,7 @@
+from django.shortcuts import render
+
+
+def test_base_template_with_no_context():
+    # base.html should be renderable with no context,
+    # so it can be easily extended.
+    render({}, 'rest_framework/base.html')