Improve type-safety, error reporting

phpcs-errors:221 phpunit-status:successful phpstan-errors:548

public/csv.php

@@ -29,7 +29,10 @@
     $selectionActive = true;
     $selectedFields = explode(",", trim($_POST['selectedFields'], ","));
 }
-else $selectionActive = false;
+else {
+    $selectionActive = false;
+    $selectedFields = [];
+}
 
 $fieldsGetter = new CsvxmlAvailableFields($lang);
 $availableFields = $fieldsGetter->getFields();