### 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)

#include "dictionary.h"
#include "core/templates/hash_map.h"
#include "core/templates/safe_refcount.h"
#include "core/variant/variant.h"
#include "core/object/class_db.h"
#include "core/object/object.h"
#include "core/variant/type_info.h"
#include "core/variant/variant_internal.h"
struct DictionaryPrivate {
	SafeRefCount refcount;
	Variant *read_only = nullptr; 
	HashMap<Variant, Variant, VariantHasher, StringLikeVariantComparator> variant_map;
};
void Dictionary::get_key_list(List<Variant> *p_keys) const {