Feature: Add "setUndefinedWhenEmpty" option to the onchange plugin

Right now, I have to manually check if the value is empty, which can be quite annoying. It would be nice if the onchange plugin had the checking built in and would just return undefined when empty.

The [isRootTextContentEmpty](https://lexical.dev/docs/api/modules/lexical_text#isroottextcontentempty) could probably be used for this