### 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)

TEST_CASE("[Dictionary] size(), empty() and clear()") {
	Dictionary map;
	CHECK(map.size() == 0);
	CHECK(map.is_empty());
	map[1] = 3;
	CHECK(map.size() == 1);
	CHECK(!map.is_empty());
	map.clear();
	CHECK(map.size() == 0);
	CHECK(map.is_empty());
}
TEST_CASE("[Dictionary] has() and has_all()") {
	Dictionary map;
	CHECK(map.has(1) == false);
	map[1] = 3;
	CHECK(map.has(1));
	Array keys;
	keys.push_back(1);
	CHECK(map.has_all(keys));
	keys.push_back(2);
	CHECK(map.has_all(keys) == false);
}
TEST_CASE("[Dictionary] keys() and values()") {
	Dictionary map;
	Array keys = map.keys();
	Array values = map.values();
	CHECK(keys.is_empty());
	CHECK(values.is_empty());
	map[1] = 3;
	keys = map.keys();