[BUGFIX] Skip any style elements with null value, but still remove them

Psalm indicates that `DOMNode::nodeValue` could be `null`,
perhaps for entirely empty `<style>` elements.

If such are encountered during pre-processing, still remove them from the DOM,
but don't append their contents to the CSS to be inlined with extra line-breaks.

psalm.baseline.xml

@@ -13,9 +13,6 @@
             }
         )]]></code>
     </InvalidArgument>
-    <PossiblyNullOperand>
-      <code><![CDATA[$styleNode->nodeValue]]></code>
-    </PossiblyNullOperand>
   </file>
   <file src="tests/Support/Constraint/CssConstraint.php">
     <DirectConstructorCall>

src/CssInliner.php

@@ -492,7 +492,9 @@ private function getCssFromAllStyleNodes(): string
 
         $css = '';
         foreach ($styleNodes as $styleNode) {
-            $css .= "\n\n" . $styleNode->nodeValue;
+            if ($styleNode->nodeValue !== null) {
+                $css .= "\n\n" . $styleNode->nodeValue;
+            }
             $parentNode = $styleNode->parentNode;
             if ($parentNode instanceof \DOMNode) {
                 $parentNode->removeChild($styleNode);