push_notifications: Handle empty rendered_messages.

This parallels fe25517295, but for mobile notifications.  It also
adds a test, which verifies that such content does not crash either
mobile or email notifications.

zerver/lib/push_notifications.py

@@ -636,7 +636,7 @@ def get_mobile_push_content(rendered_content: str) -> str:
             + "*"
         )
 
-    elem = lxml.html.fromstring(rendered_content)
+    elem = lxml.html.fragment_fromstring(rendered_content, create_parent=True)
     plain_text = process(elem)
     return plain_text