[BUGFIX] Change value from NULL to empty string for link field in test data

When increasing max length of link fields the definition for db field link in
table sys_file_reference changed to not NOT NULL. This needs to be adapted
also in the impexp functional test data.

Resolves: #68810
Related: #61944
Releases: master,6.2
Change-Id: I3c518ad865037365793a58c73ff9d1ec539410f2
Reviewed-on: http://review.typo3.org/42334


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>