Assign to null rather than use unset()

Co-authored-by: Oliver Klee <github@oliverklee.de>

Author: JakeQZ

src/RuleSet/DeclarationBlock.php

@@ -53,7 +53,7 @@ public static function parse(ParserState $parserState, ?CSSList $list = null): ?
                             $stringWrapperCharacter = $nextCharacter;
                         } elseif ($stringWrapperCharacter === $nextCharacter) {
                             if (\substr(\end($selectorParts), -1) !== '\\') {
-                                unset($stringWrapperCharacter);
+                                $stringWrapperCharacter = null;
                             }
                         }
                         break;