Enhanced get_new_version (depends on #35)

[davidmosterd]

The check for the version is now just an call to empty() as that should do?

It then assumes 'null' as its default value and tries to get a new version. Which should always work, but if it does not it returns this 'null'

Maybe for better working the fallback should return sooner or the orginal transient should be returned or the 'null' could be converted to ''

Depends on #35. Or we wait for the previous commit.

[davidmosterd]

Coud you/we look into this with some prio? As of now, the current code gives fatal errors when WP_Error is indeed triggered. Regardless of why there is a WP_Error, this really breaks the code badly.