Add st_isvalid check to imposm3 generalized tables

Fix #386

layers/waterway/update_important_waterway.sql

@@ -27,7 +27,7 @@ CREATE MATERIALIZED VIEW osm_important_waterway_linestring AS (
             ST_LineMerge(ST_Union(geometry)) AS geometry,
             name, name_en, name_de, slice_language_tags(tags) AS tags
         FROM osm_waterway_linestring
-        WHERE name <> '' AND waterway = 'river'
+        WHERE name <> '' AND waterway = 'river' AND ST_IsValid(geometry)
         GROUP BY name, name_en, name_de, slice_language_tags(tags)
     ) AS waterway_union
 );