Use is_string() rather than !== null for extra clarity, succinctl…

…y providing extra information.

Good suggestion.  I agree.  It's `null|string` (nothing else), so much clearer to test for the non-null type.

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

psalm.baseline.xml

@@ -1,10 +1,5 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <files psalm-version="5.11.0@c9b192ab8400fdaf04b2b13d110575adc879aa90">
-  <file src="src/CssInliner.php">
-    <PossiblyNullOperand>
-      <code><![CDATA[$styleNode->nodeValue]]></code>
-    </PossiblyNullOperand>
-  </file>
   <file src="tests/Support/Constraint/CssConstraint.php">
     <DirectConstructorCall>
       <code>parent::__construct()</code>

src/CssInliner.php

@@ -492,7 +492,7 @@ private function getCssFromAllStyleNodes(): string
 
         $css = '';
         foreach ($styleNodes as $styleNode) {
-            if ($styleNode->nodeValue !== null) {
+            if (\is_string($styleNode->nodeValue)) {
                 $css .= "\n\n" . $styleNode->nodeValue;
             }
             $parentNode = $styleNode->parentNode;