OutputPage: Guard jQuery.ready() against absence of jQuery

We already wrap usage of global "mediaWiki" in a condition
for window.mw (see method
ResourceLoader::makeLoaderConditionalScript) because:
1) The startup module blacklists certain
   browsers in which we won't load jquery+mediawiki.
2) It might have failed to load (for whatever reason).

Adding guard for window.jQuery for the same reasons.

Follows-up Ic3d0c937268d09, which caused a TypeError
'ready of undefined not a function' in browsers where
jquery+mediawiki isn't loaded by the startup module.

Change-Id: I9dcd8d347c6b00efe207d031a480e5b85bf78936