fix broken analytics tracking introduced in #1507 & changed default

This may not be the most elegant approach but it does fix tracking issue.
> Maybe just default to false instead of null?

Yes

> Also, I see analytics.setup is called only in bin/bower. Shouldn't we call it before calling each command inprogrammatic API? In case someone passes different analytics or interactive in two different commands?

I agree. We can deal with it in this block: https://github.com/bower/bower/blob/analytics_fix/lib/util/analytics.js#L65.