webadmin: Fix potential NPE when in CheckboxColumn

getValue(object)) in checkboxColumn.render() might return null, resulting in
an NPE when used in if. Fix the if to handle null.

Note: this bug could only occur if a checkboxColumn is not editable
*and* it's value is null.

Change-Id: I26f688984083fbd6964972e79e50960b254c547b
Signed-off-by: Vered Volansky <vvolansk@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1097754