fix event.relatedTarget fallback logic for firefox

[claytercek]

Summary

Fixed a bug where if the nativeEvent relatedTarget property was null in Firefox, it would return undefined.

Issue: #19572

Test Plan

Tested on Firefox 79, now returns null if nativeEvent is null.

Tested in the following environments to make sure behavior was unaffected and no new bugs were introduced:

• IE11 - windows
• Edge 84 - windows
• Opera 69 - mac
• Chrome 84 - mac
• Safari 13 - mac

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[gaearon]

Does the new check work as you expect for all browsers?

[claytercek]

yes! It will return null if the nativeEvent is null, otherwise it returns the relevant element.