Don't override Special:Random if $extra is set

$extra indicates some extra condition. The search engine doesn't
support this, so don't override in that case.

This fixes the RandomRootPage extension.

Note: I don't have Cirrus set up locally. Have not tested.

Bug: 66879
Change-Id: I634204e008b7dde0618425da0455be280fb20a3c
(cherry picked from commit 89981276889954160defdfa49fb474f35a62e8c0)