Postgres will fail with an error, if a table sequence is requested for
a table without auto_increment column. MySql will just return '0'.
Test the relation type before trying to persist mm relations to
prevent that error.

Resolves: #80562
Releases: master
Change-Id: Ia429e681a8ec1805996da03bd738b8347a00e40d
Reviewed-on: https://review.typo3.org/52251


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>