Fixed a flaw in the implementation of condition push-down

for materialized views and derived tables: there were no
push-down if the view was defined as union of selects
without aggregation. Added test cases with such unions.

Adjusted result files after the merge of the code for mdev-9197.

mysql-test/r/explain_json.result

@@ -485,6 +485,7 @@ EXPLAIN
       "materialized": {
         "query_block": {
           "select_id": 2,
+          "having_condition": "(cnt > 0)",
           "filesort": {
             "sort_key": "t1.a",
             "temporary_table": {