Fall back to empty string if WATCHDOG_NOTIFY_EMAIL undefined (#4457)

By default, `.env` (`mailcow.conf`) does not define `WATCHDOG_NOTIFY_EMAIL`.

Using it in `docker-compose.yml` without having it defined leads to Compose v2 displaying this warning on startup:

> WARNING: The WATCHDOG_NOTIFY_EMAIL variable is not set. Defaulting to a blank string.

Related to #4315

docker-compose.yml

@@ -464,7 +464,7 @@ services:
         - DBPASS=${DBPASS}
         - DBROOT=${DBROOT}
         - USE_WATCHDOG=${USE_WATCHDOG:-n}
-        - WATCHDOG_NOTIFY_EMAIL=${WATCHDOG_NOTIFY_EMAIL}
+        - WATCHDOG_NOTIFY_EMAIL=${WATCHDOG_NOTIFY_EMAIL:-}
         - WATCHDOG_NOTIFY_BAN=${WATCHDOG_NOTIFY_BAN:-y}
         - WATCHDOG_SUBJECT=${WATCHDOG_SUBJECT:-Watchdog ALERT}
         - WATCHDOG_EXTERNAL_CHECKS=${WATCHDOG_EXTERNAL_CHECKS:-n}