When there are no underlying tables specified for a merge table,
SHOW CREATE TABLE outputs a statement that cannot be executed. The
same is true for mysqldump (it generates dumps that cannot be
executed).

This happens because SQL parser does not accept empty UNION() clause.

This patch changes the following:
- it is now possible to execute CREATE/ALTER statement with
  empty UNION() clause.
- the same as above, but still worth noting: it is now possible to
  remove underlying tables mapping using ALTER TABLE ... UNION=().
- SHOW CREATE TABLE does not output UNION() clause if there are
  no underlying tables specified for a merge table. This makes
  mysqldump slightly smaller.