[TASK] Fix functional tests for EXT:impexp

After issue #61944 has been merged, sys_file_reference.link does
not support NULL values anymore (which is fine). However, in the
past the default has been NULL and also has been written in the
accordant functional test data assertions.

Thus, NULL assertions are removed from the test. The change of
the "size" property by 5 bytes can be explained by a difference
in serializing NULL values and serializing an empty string.

Releases: master, 6.2
Related: #61944
Change-Id: Ia8700d6b6e374412e3300f12f4d6bb341deb4cfe
Reviewed-on: http://review.typo3.org/33064
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Oliver Hader <oliver.hader@typo3.org>