Use NOREPLY_EMAIL_ADDRESS if email gateway not enabled.

This fixes a regression where missed message emails would not be sent
at all in the event that EMAIL_GATEWAY_PATTERN was unset.

The overall experience still isn't great, but it's better than crashing.

Fixes: #1411

[commit message expanded by tabbott]

zerver/lib/email_mirror.py

@@ -88,6 +88,11 @@ def get_missed_message_token_from_address(address):
 
 def create_missed_message_address(user_profile, message):
     # type: (UserProfile, Message) -> text_type
+    if settings.EMAIL_GATEWAY_PATTERN == '':
+        logging.warning("EMAIL_GATEWAY_PATTERN is an empty string, using "
+                        "NOREPLY_EMAIL_ADDRESS in the 'from' field.")
+        return settings.NOREPLY_EMAIL_ADDRESS
+
     if message.recipient.type == Recipient.PERSONAL:
         # We need to reply to the sender so look up their personal recipient_id
         recipient_id = get_recipient(Recipient.PERSONAL, message.sender_id).id