Fix isSafari() throwing on React Native (fixes #7962)

[Yonom]

In #7929 some logic was added to Firestore to check for the browser being Safari. This new check for isSafari() unexpectedly threw an exception in React Native. This PR fixes the exception by explicitly checking for an object not being undefined before using it.

Fixes #7962

[changeset-bot]

🦋 Changeset detected

Latest commit: d94a69b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 28 packages

Name	Type
@firebase/util	Patch
@firebase/analytics-compat	Patch
@firebase/analytics	Patch
@firebase/app-check-compat	Patch
@firebase/app-check	Patch
@firebase/app-compat	Patch
@firebase/app	Patch
@firebase/auth-compat	Patch
@firebase/auth	Patch
@firebase/component	Patch
@firebase/database-compat	Patch
@firebase/database-types	Patch
@firebase/database	Patch
firebase	Patch
@firebase/firestore-compat	Patch
@firebase/firestore	Patch
@firebase/functions-compat	Patch
@firebase/functions	Patch
@firebase/installations-compat	Patch
@firebase/installations	Patch
@firebase/messaging-compat	Patch
@firebase/messaging	Patch
@firebase/performance-compat	Patch
@firebase/performance	Patch
@firebase/remote-config-compat	Patch
@firebase/remote-config	Patch
@firebase/storage-compat	Patch
@firebase/storage	Patch

Not sure what this means? Click here to learn what changesets are.

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

[dconeybe]

Looks like GitHub Actions fails miserably when run in a PR from an external contributor. I've opened #7965 to run them under my name. We can use those results to gate this PR's merge then just force-merge this one.