### Godot version

v4.0.2.stable.custom_build (nixpkgs version)

### System information

NixOS

### Issue description

The [docs](https://docs.godotengine.org/en/latest/tutorials/scripting/gdscript/gdscript_basics.html#variables) seem to specify that uninitialized variables are null (as noted in this comment on the related issue: https://github.com/godotengine/godot/issues/56223#issuecomment-1175794500):
`var a # Data type is 'null' by default.`

This isn't always the case inside branches within loops. Instead, the variables can take on values from other branches.

### Related
- https://github.com/godotengine/godot/issues/56223

### Steps to reproduce

```
@tool
extends EditorScript

func _run():
	for i in 5:
		if i % 2:
			var j = i
			print("i = %s, j = %s" % [i, j])
		else:
			var k
			print("i = %s, k = %s" % [i, k])
```

Running this prints:
```
i = 0, k = <null>
i = 1, j = 1
i = 2, k = 1
i = 3, j = 3
i = 4, k = 3
```

This version also gives the exact same result:
```
@tool
extends EditorScript

func _run():
	for i in 5:
		match i % 2:
			1:
				var j = i
				print("i = %s, j = %s" % [i, j])
			0:
				var k
				print("i = %s, k = %s" % [i, k])
```

In addition, behavior similar to that described in https://github.com/godotengine/godot/issues/56223#issuecomment-1000996579 can still be reproduced as follows:
```
@tool
extends EditorScript

func _run():
	for i in 3:
		if i % 2:
			for j in range(1, 6, 3):
				pass
			print("i = %s" % i)
		else:
			var k
			var l
			var m
			print("i = %s, k = %s, l = %s, m = %s" % [i, k, l, m])
```
Which prints:
```
i = 0, k = <null>, l = <null>, m = <null>
i = 1
i = 2, k = 4, l = 7, m = (1, 6, 3)
```

### Minimal reproduction project

N/A
p_script->clearing = false;
	p_script->tool = parser->is_tool();
	if (!p_script->name.is_empty()) {
		if (ClassDB::class_exists(p_script->name) && ClassDB::is_class_exposed(p_script->name)) {
			_set_error("The class '" + p_script->name + "' shadows a native class", p_class);
			return ERR_ALREADY_EXISTS;
		}
	}
	GDScriptDataType base_type = _gdtype_from_datatype(p_class->base_type, p_script);
	int native_idx = GDScriptLanguage::get_singleton()->get_global_map()[base_type.native_type];
	p_script->native = GDScriptLanguage::get_singleton()->get_global_array()[native_idx];
	ERR_FAIL_COND_V(p_script->native.is_null(), ERR_BUG);
	
	switch (base_type.kind) {
		case GDScriptDataType::NATIVE:
			
			break;
		case GDScriptDataType::GDSCRIPT: {
			Ref<GDScript> base = Ref<GDScript>(base_type.script_type);
			if (base.is_null()) {
				return ERR_COMPILATION_FAILED;
			}
			if (main_script->has_class(base.ptr())) {
				Error err = _populate_class_members(base.ptr(), p_class->base_type.class_type, p_keep_state);
				if (err) {
					return err;
				}
			} else if (!base->is_valid()) {
				Error err = OK;
				Ref<GDScript> base_root = GDScriptCache::get_full_script(base->path, err, p_script->path);