prefer `if (s)' to `if (nil != s)' for objective-c

in objc, explicit comparison with nil isn't necessary and can produce errors like:
  comparison between distinct pointer types (...) and 'objc_object*' lacks a cast

upd: thanks Norbert Thiebaud for suggestion
  ``hitChild is a Reference so hitChild = nil was !hitChild.is() right?''

Change-Id: I105be50e5a37bb63e360622e590ec4916fa8a84f
Reviewed-on: https://gerrit.libreoffice.org/11891
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>