ref: change file_types type to nullOr(listOf str)

modules/plugins/languages/markdown.nix

@@ -119,9 +119,14 @@ in {
 
         setupOpts = mkPluginSetupOption "render-markdown" {
           file_types = lib.mkOption {
-            type = listOf (nullOr str);
-            default = [];
-            description = "List of buffer filetypes to enable this plugin in. This will cause the plugin to attach to new buffers who have any of these filetypes.";
+            type = nullOr (listOf str);
+            default = null;
+            description = ''
+              List of buffer filetypes to enable this plugin in.
+
+              This will cause the plugin to attach to new buffers who
+              have any of these filetypes.
+            '';
           };
         };
       };