mediawiki.user: Fix missing array initialization in generateRandomSessionId

mediawiki.user: Fix missing array initialization in generateRandomSessionId

Array was not properly initialized and thus browsers
that do not support Crypto API where displaying an error
on console.

The tests failed to catch this because assigning window.crypto
to `undefined` does not work (it is a read-only property). This
"fallback" test was actually testing the regular Crypto-based path
a second time.

Bug: T203275
Co-Authored-By: Timo Tijhof <krinklemail@gmail.com>
Change-Id: I8feecddf0878a739e560085f7897ebc3d8100c02