Can not set back '_subresources' param in import setting to default.

### Tested versions

4.2.2-stable (https://github.com/godotengine/godot/commit/15073afe3856abd2aa1622492fe50026c7d63dc1)

### System information

Windows10 x64

### Issue description

In 'Advanced Import Settings' modal,
When any property changes of children nodes is same as default values,
Changes are not applies to .import file (_subresources property).


### Steps to reproduce

1. Open attatched project.
2. Right click on 'test_ball.blend' in 'FileSystem' tree and choose 'Open Scene'.
3. Select 'ball1' scene tree.
4. Change 'Cast Shadow' property to 'Off' and press Reimport button.
5. Right click on 'test_ball.blend' in 'FileSystem' tree and choose 'Open Scene'.
6. Select 'ball1' scene tree.
7. Change 'Cast Shadow' property to 'On'(default value) and press Reimport button.
8. Right click on 'test_ball.blend' in 'FileSystem' tree and choose 'Open Scene'.
9. Select 'ball1' scene tree.
10. 'Cast Shadow' property has been still set 'Off'. (operation no.7 was not applied)

### Minimal reproduction project (MRP)

[subresource_bug.zip](https://github.com/godotengine/godot/files/15372090/subresource_bug.zip)

Start by opening the attached minimal reproduction project and following the Advanced Import Settings steps for test_ball.blend. Trace how the _subresources property is written when Cast Shadow is changed back to its default, then verify that reimporting and reopening the scene preserves the default value.

Written by the indexing model from the issue text.