Remove unneeded "else null" in conditions (#732)

Minor code cleanup:
SQL already returns NULL in the "WHEN" condition
if it is not matched by any of the cases.

Co-authored-by: Eva Jelinkova <evka.jelinkova@gmail.com>

layers/water/water.sql

@@ -55,7 +55,6 @@ CREATE OR REPLACE FUNCTION waterway_brunnel(is_bridge BOOL, is_tunnel BOOL) RETU
     SELECT CASE
         WHEN is_bridge THEN 'bridge'
         WHEN is_tunnel THEN 'tunnel'
-        ELSE NULL
     END;
 $$ LANGUAGE SQL IMMUTABLE STRICT;