### Godot version

4.0 (c1128e911ccd6f1e8c35646df804d894652a58f1)

### System information

Arch Linux

### Issue description

Read-only dictionaries can still have new keys added. The value for the new key isn't set (it is kept `null`).

This is caused by `Dictionary::operator[]` not checking if the key exists when it is read only mode. Accessing a key in the underlying HashMap will create if it does not exist. The actual value isn't set because it only returns a dummy reference.

### Steps to reproduce

Run the following snippet:

```gdscript
var dict = {}
dict.make_read_only()
prints("dict", dict) # {}
dict.foo = "hi"
prints("dict", dict) # { "foo": <null> }
```

### Minimal reproduction project

[DictReadOnlyAddKey.zip](https://github.com/godotengine/godot/files/10943387/DictReadOnlyAddKey.zip)

return recursive_duplicate(p_deep, 0);
}
void Dictionary::make_read_only() {
	if (_p->read_only == nullptr) {
		_p->read_only = memnew(Variant);
	}
}
bool Dictionary::is_read_only() const {
	return _p->read_only != nullptr;
}
Dictionary Dictionary::recursive_duplicate(bool p_deep, int recursion_count) const {
	Dictionary n;
	if (recursion_count > MAX_RECURSION) {
		ERR_PRINT("Max recursion reached");
		return n;
	}
	if (p_deep) {
		recursion_count++;
		for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
			n[E.key.recursive_duplicate(true, recursion_count)] = E.value.recursive_duplicate(true, recursion_count);
		}
	} else {
		for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
			n[E.key] = E.value;
		}
	}
	return n;
}
void Dictionary::operator=(const Dictionary &p_dictionary) {