Remove redundant null check

The argument can only be `DOMElement`, not `null`.

Author: jtojnar

src/Readability.php

@@ -553,7 +553,7 @@ public function prepArticle(\DOMNode $articleContent): void
      */
     public function getInnerText($e, bool $normalizeSpaces = true, bool $flattenLines = false): string
     {
-        if (null === $e || !isset($e->textContent) || '' === $e->textContent) {
+        if (!isset($e->textContent) || '' === $e->textContent) {
             return '';
         }