Return None in get_dict if key does not exist.

This is wrong from a Python point of view, but the default in the
Django Template Language.

biscuit/core/templatetags/data_helpers.py

@@ -7,4 +7,4 @@ register = template.Library()
 
 @register.filter
 def get_dict(value: Dict[Any, Any], arg: Any) -> Any:
-    return value[arg]
+    return value.get(arg, None)