Fix for the_geom does not exist

When creating triggers, expectation is to have the columns the_geom and
the_geom_webmercator even if the source table does not have any geometry
columns. Populate it in the rewrite with NULL values and right types.

scripts-available/CDB_CartodbfyTable.sql

@@ -1202,7 +1202,8 @@ BEGIN
       -- their tables to invalidate the SQL API 
       -- cache on update/insert/delete.
       geom_column_source := '';
-
+      sql := sql || ',NULL::geometry(Geometry,4326) AS ' || const.geomcol;
+      sql := sql || ',NULL::geometry(Geometry,3857) AS ' || const.mercgeomcol;
     ELSE
 
       -- table_srid = _CDB_Geometry_SRID(reloid, rec.attname);