### 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)

if (this == &p_dictionary) {
		return;
	}
	_ref(p_dictionary);
}
const void *Dictionary::id() const {
	return _p;
}
Dictionary::Dictionary(const Dictionary &p_from) {
	_p = nullptr;
	_ref(p_from);
}
Dictionary::Dictionary() {
	_p = memnew(DictionaryPrivate);
	_p->refcount.init();
}
Dictionary::~Dictionary() {
	_unref();
}