Add a sanity check for useEmulator URL.

[yuchenshi]

Hey there! So you want to contribute to a Firebase SDK?
Before you file this pull request, please read these guidelines:

Discussion

• Read the contribution guidelines (CONTRIBUTING.md).
• If this has been discussed in an issue, make sure to link to the issue here.
  If not, go file an issue about this before creating a pull request to discuss.

Testing

• Make sure all existing tests in the repository pass after your change.
• If you fixed a bug or added a feature, add a new test to cover your code.

API Changes

• At this time we cannot accept changes that affect the public API. If you'd like to help
  us make Firebase APIs better, please propose your change in an issue so that we
  can discuss it together.

[changeset-bot]

🦋 Changeset detected

Latest commit: 309cedf

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

This PR includes changesets to release 8 packages

Name	Type
@firebase/auth	Patch
firebase	Patch
@firebase/rules-unit-testing	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

[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/auth

  Type	Base (eeb1dfa)	Head (17c9758)	Diff
  browser	179 kB	179 kB	+129 B (+0.1%)
  main	179 kB	179 kB	+129 B (+0.1%)
  module	179 kB	179 kB	+129 B (+0.1%)

• firebase

  Type	Base (eeb1dfa)	Head (17c9758)	Diff
  firebase-auth.js	176 kB	176 kB	+129 B (+0.1%)
  firebase.js	831 kB	831 kB	+129 B (+0.0%)

Test Logs

• Base (eeb1dfa4): https://github.com/firebase/firebase-js-sdk/actions/runs/305244656
• Head (17c9758e): https://github.com/firebase/firebase-js-sdk/actions/runs/305337827