mediawiki.jqueryMsg: Support {{GENDER:}} with empty first parameter

(That is, gender for the user viewing the page.)

This is a bit hacky – it would be cleaner to allow empty parameters
instead of creating a separate construction for first-parameter-less
brace expansion – but this one works and the cleaner one was throwing
exceptions all over when I tried to do that.

Bug: 61320
Change-Id: I0f273951b2f8cebd50dffc6ef23520bb6fa72ccf
(cherry picked from commit 9cbf26450df16fcdb068d69e09c82eb5272bf428)