[BUGFIX] AbstractDomainObject _cleanProperties is array

Properly initialize _cleanProperties to array to prevent warnings.

Resolves: #57244
Releases: 6.2
Change-Id: I64f62cbcaf7268740af0e3c51303c49fc343136b
Reviewed-on: https://review.typo3.org/29814
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Oliver Klee
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs