ga: fixing revenue fallback for event value

Matches the behavior from the docs and client-side:

https://github.com/segmentio/analytics.js-integrations/blob/9243c32c8ab04c753640f27eee4e63a0abea1209/lib/google-analytics/index.js#L186
Merge pull request #193 from segmentio/ga/fix-revenue-fallback

ga: fixing revenue fallback for event value