[BUGFIX] Check for string before using strlen

If pi_flexform is converted to an array already, checking via strlen
produces a warning. An additional check via is_string suppresses the
warning.

Resolves: #52091
Resolves: #51684
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Change-Id: I97c41cdedb1afb28e2a9ca39e1d9cfb3921d9f47
Reviewed-on: https://review.typo3.org/24089
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn