Fix Uncaught TypeError on php 8.0 when adding a column to table creat…

…e form

"Uncaught TypeError: Expected parameter 1 to be an array, null given in tbl_columns_definition_form.inc.php:227"

Signed-off-by: William Desportes <williamdes@wdes.fr>

libraries/tbl_columns_definition_form.inc.php

@@ -217,7 +217,7 @@
         );
 
         $mime_map[$columnMeta['Field']] = array_merge(
-            $mime_map[$columnMeta['Field']],
+            isset($mime_map[$columnMeta['Field']]) ? $mime_map[$columnMeta['Field']] : [],
             array(
                 'mimetype' => Util::getValueByKey($_POST, "field_mimetype.${$columnNumber}"),
                 'transformation' => Util::getValueByKey(