Fix warnings in PHP 8.1

phpstan-baseline.neon

@@ -5636,7 +5636,7 @@ parameters:
 			path: src/PhpSpreadsheet/Writer/Xlsx/Chart.php
 
 		-
-			message: "#^Parameter \\#1 \\$rawTextData of method PhpOffice\\\\PhpSpreadsheet\\\\Shared\\\\XMLWriter\\:\\:writeRawData\\(\\) expects array\\<string\\>\\|string, int given\\.$#"
+			message: "#^Parameter \\#1 \\$rawTextData of method PhpOffice\\\\PhpSpreadsheet\\\\Shared\\\\XMLWriter\\:\\:writeRawData\\(\\) expects array\\<string\\>\\|string\\|null, int given\\.$#"
 			count: 1
 			path: src/PhpSpreadsheet/Writer/Xlsx/Chart.php
 

src/PhpSpreadsheet/Shared/XMLWriter.php

@@ -77,7 +77,7 @@ class XMLWriter extends \XMLWriter
     /**
      * Wrapper method for writeRaw.
      *
-     * @param string|string[] $rawTextData
+     * @param null|string|string[] $rawTextData
      *
      * @return bool
      */
@@ -87,6 +87,6 @@ class XMLWriter extends \XMLWriter
             $rawTextData = implode("\n", $rawTextData);
         }
 
-        return $this->writeRaw(htmlspecialchars($rawTextData));
+        return $this->writeRaw(htmlspecialchars($rawTextData ?? ''));
     }
 }