Fix: resetting/clearing the filename skipped validation

The logic to add a file extension to the filename - which runs when
editing the text was not running when resetting/clearing the value.

Resolve by calling `uiBut::func`.

Along with [0], this fixes #134101.

[0]: f2a1c8d77c

source/blender/editors/interface/interface_handlers.cc

@@ -9185,6 +9185,16 @@ void UI_context_active_but_prop_handle(bContext *C, const bool handle_undo)
      * currently this is mainly so reset defaults works for the
      * operator redo panel. */
     uiBlock *block = activebut->block;
+
+    /* There are various functions buttons may run on completion.
+     * See #ui_apply_but_funcs_after for reference, we could even call
+     * this function however it may have unforeseen consequences,
+     * see replies to: !134233. */
+
+    /* This may be needed to validate the value, see: #134101. */
+    if (activebut->func) {
+      activebut->func(C, activebut->func_arg1, activebut->func_arg2);
+    }
     if (block->handle_func) {
       block->handle_func(C, block->handle_func_arg, activebut->retval);
     }