### 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)

values = map.values();
	CHECK(int(keys[0]) == 1);
	CHECK(int(values[0]) == 3);
}
TEST_CASE("[Dictionary] Duplicate dictionary") {
	
	Dictionary k2 = build_dictionary(2, 2);
	Array k3 = build_array(3);
	Dictionary d = build_dictionary(1, build_dictionary(1, 1), k2, build_array(2), k3, 3);
	
	Dictionary deep_d = d.duplicate(true);
	CHECK_MESSAGE(deep_d.id() != d.id(), "Should create a new dictionary");
	CHECK_MESSAGE(Dictionary(deep_d[1]).id() != Dictionary(d[1]).id(), "Should clone nested dictionary");
	CHECK_MESSAGE(Array(deep_d[k2]).id() != Array(d[k2]).id(), "Should clone nested array");
	CHECK_EQ(deep_d, d);
	deep_d[0] = 0;
	CHECK_NE(deep_d, d);
	deep_d.erase(0);
	Dictionary(deep_d[1]).operator[](0) = 0;
	CHECK_NE(deep_d, d);
	Dictionary(deep_d[1]).erase(0);
	CHECK_EQ(deep_d, d);
	
	k2[0] = 0;
	CHECK_NE(deep_d, d);
	k2.erase(0);
	CHECK_EQ(deep_d, d);
	k3.push_back(0);
	CHECK_NE(deep_d, d);
	k3.pop_back();