Make use of modern ?? and ??= syntax where it makes sense

I feel like this makes the code more compact and more readable. The
syntax especially avoids any repetition.

I used a few regex patterns to find these places, but made all
changes carefully by hand, not automatically.

Change-Id: I131e69b55be0aeba32ceda45a6229cb40a3a812d

Author: thiemowmde

src/Snak/SnakList.php

@@ -255,9 +255,7 @@ private function setElement( $index, $value ) {
 			return;
 		}
 
-		if ( $index === null ) {
-			$index = $this->getNewOffset();
-		}
+		$index ??= $this->getNewOffset();
 
 		$hash = $value->getHash();
 		$this->offsetHashes[$hash] = $index;