### 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)

Array varr;
	if (_p->variant_map.is_empty()) {
		return varr;
	}
	varr.resize(size());
	int i = 0;
	for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
		varr[i] = E.key;
		i++;
	}
	return varr;
}
Array Dictionary::values() const {
	Array varr;
	if (_p->variant_map.is_empty()) {
		return varr;
	}
	varr.resize(size());
	int i = 0;
	for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
		varr[i] = E.value;
		i++;
	}
	return varr;
}
const Variant *Dictionary::next(const Variant *p_key) const {