Followup to 7b6839ac: Fix crashers during serialization

* Don't unconditionally run fromHTML and before handlers without
  checking if we have a native extension handler.

* Remove unintential implementation of `before` in Ref.php

* Hybrid tests for Cite now passes again.

* Discovered while running native parser tests and isolated those
  crashers to this.

Change-Id: I45b48b595a5aee2b8b8d00b4ebcf73a5ea7bc8a3

src/Parsoid/Ref.php

@@ -164,9 +164,4 @@ class Ref extends ExtensionTag {
 		);
 		return $startTagSrc . $src . '</' . $dataMw->name . '>';
 	}
-
-	/** @inheritDoc */
-	public function before( DOMElement $node, DOMNode $otherNode, SerializerState $state ): ?array {
-		return null;
-	}
 }