UnityAds adapter: route null/empty bidding token to failure

collectSignalsForRequestParameters coerced a null/empty token into an
empty success, so the bidder billed Unity as a no-fill. Route null/empty
tokens to the completion handler with a failure (new error code
GADMAdapterUnityErrorEmptyBiddingToken).

Updated the existing nil token test and added a test for the empty
token case.

Author: vishaldhiman-unity