[BUGFIX] Follow-up: Add stdWrap for _DEFAULT_PI_VARS

Fix a broken unit test: Creating default object from empty value

Resolves: #49314
Related: #22045
Releases: 6.2
Change-Id: Iad2582f1949202441086c1d38605be18ac03a20c
Reviewed-on: https://review.typo3.org/21563
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters