### 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 d;
	d[d] = 1;
	
	ERR_PRINT_OFF;
	d.hash();
	ERR_PRINT_ON;
	
	d.clear();
}
#endif
TEST_CASE("[Dictionary] Empty comparison") {
	Dictionary d1;
	Dictionary d2;
	
	CHECK_EQ(d1, d2);
	CHECK_FALSE(d1 != d2);
}
TEST_CASE("[Dictionary] Flat comparison") {
	Dictionary d1 = build_dictionary(1, 1);
	Dictionary d2 = build_dictionary(1, 1);
	Dictionary other_d = build_dictionary(2, 1);
	
	CHECK_EQ(d1, d1); 
	CHECK_FALSE(d1 != d1);
	CHECK_EQ(d1, d2); 
	CHECK_FALSE(d1 != d2);
	CHECK_NE(d1, other_d); 
	CHECK_FALSE(d1 == other_d);
}
TEST_CASE("[Dictionary] Nested dictionary comparison") {