### 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 (p_key == nullptr) {
		
		if (_p->variant_map.begin()) {
			return &_p->variant_map.begin()->key;
		}
		return nullptr;
	}
	HashMap<Variant, Variant, VariantHasher, StringLikeVariantComparator>::Iterator E = _p->variant_map.find(*p_key);
	if (!E) {
		return nullptr;
	}
	++E;
	if (E) {
		return &E->key;
	}
	return nullptr;
}
Dictionary Dictionary::duplicate(bool p_deep) const {