Restore registerRootListener null call

[zurfyx]

Restores the null call I removed in #6389, unfortunately it's a breaking change. Products are using this to do the teardown. For example:

if (prevElement !== null) {
    prevElement.removeEventListener('click', handleClick);
    prevElement.removeEventListener('pointerdown', handlePointerDown);
  }

We may need to introduce a new function/signature and always migrate to it. If we were to mimic React this would be intuitive and wouldn't need to return a fake null rootElement:

registerRootListener(prevElement, nextElement) {
  
  return () => {
    if (prevElement !== null) {
      prevElement.removeEventListener(...);
  }

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Path	Size
lexical - cjs	28.71 KB (0%)
lexical - esm	28.56 KB (0%)
@lexical/rich-text - cjs	37.17 KB (0%)
@lexical/rich-text - esm	28.13 KB (0%)
@lexical/plain-text - cjs	35.82 KB (0%)
@lexical/plain-text - esm	25.48 KB (0%)
@lexical/react - cjs	39.09 KB (0%)
@lexical/react - esm	29.48 KB (0%)

[ivailop7]

that didn't last long

[zurfyx]

that didn't last long

@ivailop7 haha right, but it's worth pointing out I didn't revert the other PR fully, the nullable condition still remains