[BUGFIX] Check if `$rows` is an array before iteration

In case `$rows` in the method `DataHandler::copySpecificPage()` is null,
the foreach loop will throw an error.
`$rows` may become null if an extension brings TCA but the table does not
have `uid` field, for example.

It's now checked whether the result being iterated is an array, otherwise
an error is logged.

Resolves: #74125
Releases: master, 7.6
Change-Id: I20cb101155632309b9e08600fcd33e655c1c9d2f
Reviewed-on: https://review.typo3.org/48311


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>