Fix bug where we raised StopIteration in a generator

This made python 3.7 unhappy

synapse/rest/media/v1/preview_url_resource.py

@@ -596,10 +596,13 @@ def _iterate_over_text(tree, *tags_to_ignore):
     # to be returned.
     elements = iter([tree])
     while True:
-        el = next(elements)
+        el = next(elements, None)
+        if el is None:
+            return
+
         if isinstance(el, string_types):
             yield el
-        elif el is not None and el.tag not in tags_to_ignore:
+        elif el.tag not in tags_to_ignore:
             # el.text is the text before the first child, so we can immediately
             # return it if the text exists.
             if el.text: