Added check to return a default language if the language header is empty...

I found a few new cases that were causing errors on modern.IE. I've outlined those those below.

- [ ] "" -> "en-us"
- [ ] "en-us, en;q=0.7, *;q=0.01" -> "en-us"
- [ ] "en-AU,geo;q=0.7,ru;q=0.3" -> "en-au"

I also modified the code to not return null if a language could be parsed out of the header since the previously failing cases above are still able to be parsed for languages.