[13.x] Fix trialEndsAt more closely represents onTrial

[benjamindoe]

Closes #1128

The trialEndsAt returns the trial_ends_at attribute by default if there are no arguments passed through and the user is indeed on a generic trial. Without the onGenericTrial check, the function will always return trial_ends_at if no arguments were passed through.

The trialEndsAt function also returns trial_ends_at as a fallback because there may be a use case that an application expects a historical date from trialEndsAt.

[driesvints]

@benjamindoe Can you confirm this doesn't breaks other tests?

[benjamindoe]

All tests pass locally. Tested on 8 and 7.4.

[driesvints]

Thanks @benjamindoe