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/landcover/landcover.sql

@@ -12,7 +12,6 @@
 CREATE OR REPLACE FUNCTION landcover_class(subclass VARCHAR) RETURNS TEXT AS $$
     SELECT CASE
         %%FIELD_MAPPING: class %%
-        ELSE NULL
     END;
 $$ LANGUAGE SQL IMMUTABLE;