[BUGFIX] Allow empty values in start/stop filter of belog

One can limit records in the backend log module to a custom period
of time using the "User defined" mode and a start and stop field.

Leaving these fields empty is now supported in the corresponding
domain model setters by adding default values to the DateTime
arguments.

Resolves: #53975
Releases: 6.1, 6.2
Change-Id: I944322bb3747a29fab33c3d32e17dd070c7cb6db
Reviewed-on: https://review.typo3.org/26723
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters