Use is_string rather than isset

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

Author: JakeQZ

src/RuleSet/DeclarationBlock.php

@@ -49,7 +49,7 @@ public static function parse(ParserState $parserState, ?CSSList $list = null): ?
                     case '\'':
                         // The fallthrough is intentional.
                     case '"':
-                        if (!isset($stringWrapperCharacter)) {
+                        if (!\is_string($stringWrapperCharacter)) {
                             $stringWrapperCharacter = $nextCharacter;
                         } elseif ($stringWrapperCharacter === $nextCharacter) {
                             if (\substr(\end($selectorParts), -1) !== '\\') {