DropdownInputWidget: Unbreak setting 'value' via config options
DropdownInputWidget: Unbreak setting 'value' via config options

This change reveals that #setValue did not correctly refuse to set
invalid options. (Rather than fall back to one of the valid values,
we would set the value to ''.)

Also prevent exceptions when the menu has zero options.

Bug: T166783
Change-Id: I599ce039190b7b2ab6b10a13b14b19a5b1e38625

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