### 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
can_continue = true;
	SuiteNode *suite = alloc_node<SuiteNode>();
	if (n_for->variable) {
		const SuiteNode::Local &local = current_suite->get_local(n_for->variable->name);
		if (local.type != SuiteNode::Local::UNDEFINED) {
			push_error(vformat(R"(There is already a %s named "%s" declared in this scope.)", local.get_name(), n_for->variable->name), n_for->variable);
		}
		suite->add_local(SuiteNode::Local(n_for->variable, current_function));
	}
	n_for->loop = parse_suite(R"("for" block)", suite);
	n_for->loop->is_loop = true;
	complete_extents(n_for);
	
	can_break = could_break;
	can_continue = could_continue;
	return n_for;
}
GDScriptParser::IfNode *GDScriptParser::parse_if(const String &p_token) {
	IfNode *n_if = alloc_node<IfNode>();
	n_if->condition = parse_expression(false);
	if (n_if->condition == nullptr) {
		push_error(vformat(R"(Expected conditional expression after "%s".)", p_token));
	}
	consume(GDScriptTokenizer::Token::COLON, vformat(R"(Expected ":" after "%s" condition.)", p_token));
	n_if->true_block = parse_suite(vformat(R"("%s" block)", p_token));
	n_if->true_block->parent_if = n_if;
	if (n_if->true_block->has_continue) {
		current_suite->has_continue = true;
	}
	if (match(GDScriptTokenizer::Token::ELIF)) {