Only return the latest modification (Bug #13113).
Fix testing for unset old properties.

Fixes logging all properties that are empty, even if not changed. Introduces
with request #10249.
Don't pile up modifications in history log.

The changes introduced for request #10249 did accumulate all event
modifications in the history log, which eventually lead to bug #13113. Until
Horde_History doesn't scale better for large user numbers, we should not log
even more stuff. Especially if we don't even use this information anywhere.