[mw.util] bug fix and minor clean up

[mw.util] bug fix and minor clean up
* Fixes bug 34603 (Patch by Rainer <Rainer@Rillke.eu>) and other instances of the same bug
-- Use 'util' instead of 'this' to allow re-usage of the method in a different context
* Use ternary operator instead of the default operator in mw.util.wikiGetlink to check it for type instead of thruthy-ness (so that falsy-values such as '' work, and thruthy non-string values such as objects fail).