SelectWidget: Optimize #getRelativeSelectableItem without filter
SelectWidget: Optimize #getRelativeSelectableItem without filter

Avoid function calls when no 'filter' was given.

Change-Id: I79d3734bc1ad6bee24a1617db44583c66df4de1c

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui