🐛 Fix bug in migration

django-modeltranslation fields can be None, which breaks the re.sub
usage which expects a string.

src/openforms/emails/migrations/0003_update_summary_tags.py

@@ -9,7 +9,9 @@
 SUMMARY_REGEX = r"{%\s*summary\s*%}"
 
 
-def replace_tag(tpl: str) -> str:
+def replace_tag(tpl: str | None) -> str | None:
+    if tpl is None:
+        return None
     return re.sub(
         SUMMARY_REGEX,
         "{% confirmation_summary %}",