fix(ios): handle nil messaging token

[TPXP]

Description

On iOS, calling [FIRInstanceID instanceID] deleteIDWithHandler causes a crash. I quite like this method since it enables me to drop all tokens generated until now, and make sure we won't send any more messages to this device (use case: user logout).

Related issues

Not reported.

Release Summary

[messaging] Fixed a crash if the fetched token is null (typically when deleting the cloud messaging instance).

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • iOS
• My change includes tests: NO (but maybe it should!)
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

You can try to call [FIRInstanceID instanceID] deleteIDWithHandler in your app code (exposing it as a JS API would be welcome by the way!).

Thanks a lot for this project!

[CLAassistant]

All committers have signed the CLA.

[Salakar]

Re the JS api, its already available through the Instance ID package: https://rnfirebase.io/reference/iid#delete

[Salakar]

LGTM if this fixes a crash. Though one minor point; should we not be sending null tokens to JS so you know when a token has been invalidated? I think Android sends null - the JS SDK Web types don't really indicate what should happen here and is just typed as any 🙈

EDIT: shipped, 'fix it in post' as they say if this should actually emit nulls