Ignore empty text nodes when searching for existing paragraphs

See https://www.woltlab.com/community/thread/314964/

Author: dtdesign

wcfsetup/install/files/lib/system/html/input/node/HtmlInputNodeProcessor.class.php

@@ -351,11 +351,19 @@ protected function fixDom()
             }
         }
 
-        $appendToPreviousParagraph = static function ($node) {
-            /** @var \DOMElement $paragraph */
-            $paragraph = $node->previousSibling;
+        $appendToPreviousParagraph = static function (\DOMNode $node) {
+            $paragraph = $node;
+            while ($paragraph = $paragraph->previousSibling) {
+                if ($paragraph instanceof \DOMText) {
+                    if (!\str_contains($paragraph->textContent, ' ') && StringUtil::trim($paragraph->textContent) === '') {
+                        continue;
+                    }
+                }
+
+                break;
+            }
 
-            if (!$paragraph || $paragraph->nodeName !== 'p') {
+            if ($paragraph === null || $paragraph->nodeName !== 'p') {
                 $paragraph = $node->ownerDocument->createElement('p');
                 $node->parentNode->insertBefore($paragraph, $node);
             }