Fix AppCheck getToken docs text

[hsubox76]

getToken() docs text inaccurately said that it returns null if no token is available. Fixed text to describe behavior accurately.

[changeset-bot]

⚠️ No Changeset found

Latest commit: ac5fc45

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (c6a8851) and merge commit (f45161e).

Test Logs

• Base (c6a8851): https://github.com/firebase/firebase-js-sdk/actions/runs/10509900088
• Merge (f45161e): https://github.com/firebase/firebase-js-sdk/actions/runs/10530959067

1. https://storage.googleapis.com/firebase-sdk-metric-reports/ebVDClUWpo.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (c6a8851) and merge commit (f45161e).

Test Logs

• Base (c6a8851): https://github.com/firebase/firebase-js-sdk/actions/runs/10509900088
• Merge (f45161e): https://github.com/firebase/firebase-js-sdk/actions/runs/10530959067

1. https://storage.googleapis.com/firebase-sdk-metric-reports/l3aQTreKbw.html