[CLEANUP] Stupid array assignment

There are two places where a new item is assigned
to an array with `$myArray[] .= ''`.
This would do the same as `$myArray[] = ''` because
it always adds a new item to the array.

Resolves: #60101
Releases: 6.3
Change-Id: I122b0fe80b1ac026c0c9f353446bb48834eba74e
Reviewed-on: https://review.typo3.org/31321
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Nicole Cordes
Tested-by: Nicole Cordes