Skip calling set custom value if nothing has changed.

The call will trigger a setup of the local help manager,
which is rather heavy and should be avoided if possible.

Change-Id: I03fbe6a30fba6ac48361c7e2b5aa6d45dad5126b
Reviewed-by: Niels Weber <niels.2.weber@nokia.com>
Reviewed-by: Karsten Heimrich <karsten.heimrich@nokia.com>