Allow users to submit empty content when editing messages.

The closest we have to deleting a message. Was original behavior, but was
changed in 09754c9.

zerver/tests/test_messages.py

@@ -984,7 +984,9 @@ class EditMessageTest(ZulipTestCase):
             'message_id': msg_id,
             'content': ' '
         })
-        self.assert_json_error(result, "Content can't be empty")
+        self.assert_json_success(result)
+        content = Message.objects.filter(id=msg_id).values_list('content', flat = True)[0]
+        self.assertEqual(content, "(deleted)")
 
     def test_edit_message_content_limit(self):
         # type: () -> None

zerver/views/messages.py

@@ -911,7 +911,7 @@ def update_message_backend(request, user_profile,
     if content is not None:
         content = content.strip()
         if content == "":
-            raise JsonableError(_("Content can't be empty"))
+            content = "(deleted)"
         content = truncate_body(content)
 
         # We exclude UserMessage.flags.historical rows since those