UNION ALL queries are a subject of optimization introduced in MDEV-334
when creation of a temporary table is skipped.
While there is a check for this optimization in Explain_union::print_explain()
there was no such in Explain_union::print_explain_json(). This resulted in
printing irrelevant data like:
  "union_result": {
    "table_name": "<union2,3>",
    "access_type": "ALL",
    "r_loops": 0,
    "r_rows": null
in case when creation of the temporary table was actually optimized out.
This commits adds a check whether the temporary table was actually created
during the UNION ALL processing and eliminates printing of the irrelevant data.