[TYPO3-core] Bugfix to Bidi-MM: Set tablenames field only when necessary

[Michael Stucki]

Hi Ingmar,

> the bidirectional MM relation feature by Sebastian and me had a little
> quirk:
> It tried to set the "tablenames" field in writeMM() always, even if it
> was a simple two-table relationship, which caused the insert queries to
> not work in such cases, because the MM table didn't have the tablenames
> field.
>
> The attached patch fixes that problem by setting the tablenames field
> only when necessary.

Seems all obvious, so I didn't check if it really works.

+1 anyway.

Michael