Fix JS error in GA4 event tracking (Fixes #14177)

[alexgibson]

One-line summary

Fixes an error that is caused when trying to send an event and getEventFromUrl() returns a boolean instead of an object.

Issue / Bugzilla link

#14177

Testing

1. Open http://localhost:8000/en-US/firefox/122.0/releasenotes/
2. Open the web console and make sure you have "Persist logs" checked.
3. Scroll to the bottom of the page where it says "Get the most recent version" and click "Download Firefox"
4. Verify there is no longer a JS error on click.

[craigcook]

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