fix access error

Fix the error "Trying to access array offset on value of type null"

Author: brainfoolong

src/PhpSpreadsheet/Reader/Xlsx/Styles.php

@@ -163,7 +163,7 @@ private function readStyle(Style $docStyle, $style)
             self::readBorderStyle($docStyle->getBorders(), $style->border);
         }
 
-        if (isset($style->alignment)) {
+        if (isset($style->alignment->alignment)) {
             self::readAlignmentStyle($docStyle->getAlignment(), $style->alignment);
         }