Complement: use SQLite by default (#13075)

If no database is configured explicitly, use sqlite.

This means that you don't have to pass any variables into the image.

docker/complement/conf/start_for_complement.sh

@@ -31,7 +31,7 @@ case "$SYNAPSE_COMPLEMENT_DATABASE" in
     export START_POSTGRES=true
     ;;
 
-  sqlite)
+  sqlite|"")
     # Configure supervisord not to start Postgres, as we don't need it
     export START_POSTGRES=false
     ;;