[BUGFIX] Properly check existence of array item

The flexform converter in ContentObjectRenderer tries to access
an array element on a non-array.

Check existence with isset() first.

Resolves: #57809
Releases: 6.2, 6.1
Change-Id: I8e6111afee3a639b3077dc59bc2e32b72fa12f5c
Reviewed-on: https://review.typo3.org/29892
Reviewed-by: Markus Klein
Tested-by: Markus Klein