[BUGFIX] TCA tree with maxitems=1 doesn't work

* Check if value of the parent field is neither empty nor 0, 
  because t3lib_transferData returns 0 if the database field 
  is empty and maxitems=1
* Only check if the selected nodes reached the maxitems limit
  if a node is checked, not if one is unchecked.

Change-Id: I89efc539cf720a6798d244e0a499d6bebff32d9f
Resolves: #25347
Reviewed-on: http://review.typo3.org/1211
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Marco Huber
Tested-by: Marco Huber
Reviewed-by: Andreas Wolf