InCell: correct for Z/T recording oddities in InCell 6000

Fixes #12416.
InCell: prevent NPE if the imaging mode is not specified
Merge remote-tracking branch 'origin/dev_5_0' into rebased/dev_5_0/12416
Merge pull request #1216 from melissalinkert/rebased/dev_5_0/12416

Fix Z and T size calculation for InCell 6000 data (rebased onto dev_5_0)