[BUGFIX] Fix old XCLASS deprecation log

The correct check for the old xclass register usage is to test for array
entry count greater than zero. Furthermore the comments have been improved.

Change-Id: I0ce923c3bb220872dfc6482e963240271408d9ec
Resolves: #36703
Releases: 6.0
Reviewed-on: http://review.typo3.org/10870
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn