### Godot version

4.2.x master [5df98679672c12d96b4ac4d96ee17f1559207401]

### System information

Linux - GCC g++ 9.4.0 - Ubuntu 20.04 LTS

### Issue description

I somehow managed to get my Windows system into a broken state were the driver reports 145 extensions, but extension `#144` is an empty string.

Therefore Godot will crash while initializing Vulkan and looking for requested device extensions.

### Steps to reproduce

Execute the following code:

```cpp
HashMap<CharString, bool> requested_device_extensions;

CharString extension_name("");
CharString extension_name2("Hello");

requested_device_extensions.insert("HashMap must not be empty", true);

if (requested_device_extensions.has(extension_name2))
	printf("Has Hello\n");
if (requested_device_extensions.has(extension_name))
	printf("Has <empty>\n");
```

It will crash before <empty>. Callstack:

                                                                                                                                                                                                
```                                                                                                                                                                                                
1 hash_djb2                                                                                                                                                   hashfuncs.h        64   0x4a135d6 
2 HashMapHasherDefault::hash                                                                                                                                  hashfuncs.h        313  0x4a1decd 
3 HashMap<CharString, bool, HashMapHasherDefault, HashMapComparatorDefault<CharString>, DefaultTypedAllocator<HashMapElement<CharString, bool>>>::_hash       hash_map.h         85   0x4a1dbd9 
4 HashMap<CharString, bool, HashMapHasherDefault, HashMapComparatorDefault<CharString>, DefaultTypedAllocator<HashMapElement<CharString, bool>>>::_lookup_pos hash_map.h         106  0x4a1d8d7 
5 HashMap<CharString, bool, HashMapHasherDefault, HashMapComparatorDefault<CharString>, DefaultTypedAllocator<HashMapElement<CharString, bool>>>::has         hash_map.h         310  0x4a14788 
6 Main::setup                                                                                                                                                 main.cpp           2115 0x4a088d3 
7 main                                                                                                                                                        godot_linuxbsd.cpp 62   0x497f058 
```

The reason is that CharString decided to not create a pointer at all to save memory. Hence when hash_djb2 tries to read the first byte, it crashes.

### Minimal reproduction project

N / A
union {
		double d;
		uint64_t i;
	} u;
	
	if (p_in == 0.0f) {
		u.d = 0.0;
	} else if (Math::is_nan(p_in)) {
		u.d = NAN;
	} else {
		u.d = p_in;
	}
	return ((p_prev << 5) + p_prev) + u.i;
}
static _FORCE_INLINE_ uint64_t hash_djb2_one_64(uint64_t p_in, uint64_t p_prev = 5381) {
	return ((p_prev << 5) + p_prev) ^ p_in;
}
template <class T>
static _FORCE_INLINE_ uint64_t hash_make_uint64_t(T p_in) {
	union {
		T t;
		uint64_t _u64;
	} _u;
	_u._u64 = 0; 
	_u.t = p_in;
	return _u._u64;
}
template <class T>
class Ref;