Fix bug for tables without geom or with no potential overviews

If the table doesn't have geometries but the createoverviews function is
called, the current geometry type checks won't work because "null" will
not give a boolean value in the type comparisons.

Also, if the createoverviews function is called over a simple table with
would not require overviews according to the strategies it is handled
correctly.

scripts-available/CDB_Overviews.sql

@@ -226,6 +226,12 @@ AS $$
         WHEN internal_error THEN
           -- Get stats and execute again
           EXECUTE format('ANALYZE %1$s', reloid);
+
+          -- We check the geometry type in case the error is due to empty geometries
+          IF _CDB_GeometryTypes(reloid) IS NULL THEN
+            RETURN NULL;
+          END IF;
+
           EXECUTE ext_query INTO ext;
     END;
 
@@ -622,7 +628,7 @@ AS $$
     table_name TEXT;
   BEGIN
     SELECT _CDB_GeometryTypes(reloid) INTO gtypes;
-    IF array_upper(gtypes, 1) <> 1 OR gtypes[1] <> 'ST_Point' THEN
+    IF gtypes IS NULL OR array_upper(gtypes, 1) <> 1 OR gtypes[1] <> 'ST_Point' THEN
       -- This strategy only supports datasets with point geomety
       RETURN NULL;
       RETURN 'x';
@@ -738,6 +744,10 @@ BEGIN
   -- Determine the referece zoom level
   EXECUTE 'SELECT ' || quote_ident(refscale_strategy::text) || Format('(''%s'', %s);', reloid, tolerance_px) INTO ref_z;
 
+  IF ref_z < 0 OR ref_z IS NULL THEN
+    RETURN NULL;
+  END IF;
+
   -- Determine overlay zoom levels
   -- TODO: should be handled by the refscale_strategy?
   overview_z := ref_z - 1;