Fix bug with forcing a user to exist on request

Allow for testing views without having to mock a user on the request.

CONTRIBUTORS.rst

@@ -163,6 +163,7 @@ Contributors
 * Jesse Legg
 * Tim Leguijt
 * Luiz Boaretto
+* Jonathon Moore
 
 Translators
 ===========

wagtail/wagtailadmin/templatetags/wagtailuserbar.py

@@ -36,8 +36,14 @@ def wagtailuserbar(context, position='bottom-right'):
     except KeyError:
         return ''
 
+    # Don't render without a user because we can't check their permissions
+    try:
+        user = request.user
+    except AttributeError:
+        return ''
+
     # Don't render if user doesn't have permission to access the admin area
-    if not request.user.has_perm('wagtailadmin.access_admin'):
+    if not user.has_perm('wagtailadmin.access_admin'):
         return ''
 
     # Don't render if this is a preview. Since some routes can render the userbar without going through Page.serve(),