### 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)

Dictionary map;
	map[4] = 3;
	Variant val = map.get_key_at_index(0);
	CHECK(int(val) == 4);
	map[3] = 1;
	val = map.get_key_at_index(0);
	CHECK(int(val) == 4);
	val = map.get_key_at_index(1);
	CHECK(int(val) == 3);
}
TEST_CASE("[Dictionary] getptr()") {
	Dictionary map;
	map[1] = 3;
	Variant *key = map.getptr(1);
	CHECK(int(*key) == 3);
	key = map.getptr(2);
	CHECK(key == nullptr);
}
TEST_CASE("[Dictionary] get_valid()") {
	Dictionary map;
	map[1] = 3;
	Variant val = map.get_valid(1);
	CHECK(int(val) == 3);
}
TEST_CASE("[Dictionary] get()") {
	Dictionary map;
	map[1] = 3;
	Variant val = map.get(1, -1);
	CHECK(int(val) == 3);
}