Rewarded video: isReady crashes if not preceded by requestAd

[eranbes]

At least in Android, calling AdMobRewarded.isReady(cb) crashes the app unless AdMobRewarded.requestAd() was called before.
It would be beneficial to be able to ask if video is present without need to do requestAd first, though.

[akyGit]

PR has been created: #421