fix: avoid null array offset in rex_list::getSortColumn() under PHP 8.5 (#6586)

When the `list` request parameter matches the list name but no `sort`
parameter is present, `$sortColumn` is null. Passing it to
hasColumnOption() results in isset($this->columnOptions[null][...]).

Author: tbaddade

.tools/psalm/baseline.xml

@@ -4370,7 +4370,6 @@
       <code><![CDATA[$columnLayout]]></code>
       <code><![CDATA[$column['width'] ?? null]]></code>
       <code><![CDATA[$sortColumn]]></code>
-      <code><![CDATA[$sortColumn]]></code>
       <code><![CDATA[$this->getValue($column)]]></code>
       <code><![CDATA[$this->getValue($columnName)]]></code>
     </PossiblyNullArgument>

redaxo/src/core/lib/list.php

@@ -977,7 +977,7 @@ public function getSortColumn($default = null)
     {
         if (rex_request('list', 'string') == $this->getName()) {
             $sortColumn = rex_request('sort', 'string', $default);
-            if ($this->hasColumnOption($sortColumn, REX_LIST_OPT_SORT)) {
+            if (null !== $sortColumn && $this->hasColumnOption($sortColumn, REX_LIST_OPT_SORT)) {
                 return $sortColumn;
             }
         }