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/poi/poi_stop_agg.sql

@@ -3,8 +3,7 @@ CREATE MATERIALIZED VIEW osm_poi_stop_centroid AS (
   SELECT
       uic_ref,
       count(*) as count,
-			CASE WHEN count(*) > 2 THEN ST_Centroid(ST_UNION(geometry))
-			ELSE NULL END AS centroid
+      CASE WHEN count(*) > 2 THEN ST_Centroid(ST_UNION(geometry)) END AS centroid
   FROM osm_poi_point
 	WHERE
 		nullif(uic_ref, '') IS NOT NULL