fix(ngJq): properly detect when `ng-jq` is empty

Previously, even when `ng-jq` was empty (which should force the use of
jqLite), Angular tried to find jQuery on `window['']`. If it didn't find
anything there, it would fall back to jqLite (as expected).

Nonetheless, trying to access `window['']` calls `getElementById('')`,
which issues a warning in Firefox (maybe others).

This fix properly detects when `ng-jq` is empty and avoids trying to
access `window['']`.

Fixes #12741