_cdb_estimated_extent: Adapt to ST_EstimatedExtent change

The behaviour was changed in Postgis 1.5.4 to return NULL instead of throwing

NEWS.md

@@ -1,5 +1,6 @@
 0.24.0 (2018-XX-XX)
 * Travis: Test with PostgreSQL 9.5 and 10.
+* _cdb_estimated_extent: Fix bug with ST_EstimatedExtent interaction.
 
 0.23.2 (2018-07-19)
 * Fix `CDB_QueryTablesText` with parenthesized queries (#335)

scripts-available/CDB_Overviews.sql

@@ -93,11 +93,8 @@ AS $$
       table_id.schema_name, table_id.table_name, 'the_geom_webmercator'
     );
 
-    BEGIN
-      EXECUTE ext_query INTO ext;
-    EXCEPTION
-        -- This is the typical ERROR: stats for "mytable" do not exist
-        WHEN internal_error THEN
+    EXECUTE ext_query INTO ext;
+    IF ext IS NULL THEN
           -- Get stats and execute again
           EXECUTE format('ANALYZE %1$s', reloid);
 
@@ -107,7 +104,7 @@ AS $$
           END IF;
 
           EXECUTE ext_query INTO ext;
-    END;
+    END IF;
 
     RETURN ext;
   END;