HTMLMultiSelectField: Improve compat with GET forms with no wpFormIdentifier

HTMLMultiSelectField: Improve compat with GET forms with no wpFormIdentifier

Now it behaves more like HTMLCheckField: if there is clearly a value
submitted, it will load it.

Setting wpFormIdentifier is still needed for forms with default-on
checkboxes (without form identifier, it is impossible to distinguish a
page view without form submission, and form submission with default-on
checkbox unchecked).

In particular, this fixes the 'Show additional logs' checkboxes on
Special:Log: they no longer get unchecked after form submission.

Change-Id: Ief74a7e424b37ccd44759133b3cb8665275314a6