### Godot version

4.0.2

### System information

Win10

### Issue description

Assigning a `MultiplayerSpawner.spawn_function` with a `@tool` script causes it to corrupt a `.tscn` file. 

The resulting file will have a line that looks like this:

```
spawn_function = 
```

Which cases the file to fail to load. Removing that line fixes the file.

### Steps to reproduce

```python
@tool
extends MultiplayerSpawner

func _ready():
	spawn_function = spawnerFunction
	
func spawnerFunction():
	pass
```

Create this minimum GDScript, add it to a new `MultiplayerSpawner`, then save that file. You might need to to re-load the engine, modify the scene, and re-save to trigger the bug.

When the scene is saved, you'll see something like the below, which will refuse to load with this error message:
![image](https://user-images.githubusercontent.com/1642297/233248706-f87e35ce-76fd-4002-ad18-654ba197d468.

### Minimal reproduction project

```
[gd_scene load_steps=2 format=3 uid="uid://is84hf1r3l7u"]

[sub_resource type="GDScript" id="GDScript_hbm3l"]
script/source = "@tool
extends MultiplayerSpawner



func _ready():
	spawn_function = spawnerFunction
	

func spawnerFunction():
	pass
"

[node name="BrokenSpawner" type="MultiplayerSpawner"]
spawn_path = NodePath(".")
spawn_function = 
script = SubResource("GDScript_hbm3l")
```
if (!get_multiplayer()->has_multiplayer_peer() || !is_multiplayer_authority()) {
		return;
	}
	if (tracked_nodes.has(p_node->get_instance_id())) {
		return;
	}
	const Node *parent = get_spawn_node();
	if (!parent || p_node->get_parent() != parent) {
		return;
	}
	int id = find_spawnable_scene_index_from_path(p_node->get_scene_file_path());
	if (id == INVALID_ID) {
		return;
	}
	const String name = p_node->get_name();
	ERR_FAIL_COND_MSG(name.validate_node_name() != name, vformat("Unable to auto-spawn node with reserved name: %s. Make sure to add your replicated scenes via 'add_child(node, true)' to produce valid names.", name));
	_track(p_node, Variant(), id);
}
NodePath MultiplayerSpawner::get_spawn_path() const {