Raise a ValueError in localtime filter if no datetime

bolt/templates/jinja/filters.py

@@ -11,7 +11,7 @@ def localtime_filter(value, timezone=None):
     if not value:
         # Without this, we get the current localtime
         # which doesn't make sense as a filter
-        return None
+        raise ValueError("localtime filter requires a datetime")
     return localtime(value, timezone)