[BUGFIX] Avoid exceptions on NULL pi_flexform

[YKWeyer]

Issue

The default value of the pi_flexform column in the database is NULL.

It is possible for editors to reach that state e.g. by changing the CType of an existing ContentElement, but GeneralUtility::xml2array (used in convertFlexFormContentToArray) expects a string, causing PHP exceptions in the backend.

How to reproduce

1. Create a new ContentElement of type Text or Header
2. Change the CType field to Powermail
3. When the Refresh required popup appears, select Save and refresh
4. Close the editing of the ContentElement

Opening the page view in the backend will return a PHP exception:

(1/1) TypeError

TYPO3\CMS\Core\Utility\GeneralUtility::xml2array(): Argument #1 ($string) must be of type string, null given, called in /app/vendor/typo3/cms-core/Classes/Service/FlexFormService.php on line 40

Changes

When pi_flexform is null, then the rendering of the preview is skipped. Only an error message will be displayed in the backend: