### 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
while (check(GDScriptTokenizer::Token::DEDENT) || check(GDScriptTokenizer::Token::INDENT) || check(GDScriptTokenizer::Token::NEWLINE)) {
			current = tokenizer.scan(); 
		}
		tokenizer.pop_expression_indented_block();
	}
	current_function = previous_function;
	in_lambda = previous_in_lambda;
	lambda->function = function;
	
	can_break = could_break;
	can_continue = could_continue;
	return lambda;
}
GDScriptParser::ExpressionNode *GDScriptParser::parse_type_test(ExpressionNode *p_previous_operand, bool p_can_assign) {
	TypeTestNode *type_test = alloc_node<TypeTestNode>();
	reset_extents(type_test, p_previous_operand);
	update_extents(type_test);
	type_test->operand = p_previous_operand;
	type_test->test_type = parse_type();
	complete_extents(type_test);
	if (type_test->test_type == nullptr) {
		push_error(R"(Expected type specifier after "is".)");
	}
	return type_test;
}
GDScriptParser::ExpressionNode *GDScriptParser::parse_yield(ExpressionNode *p_previous_operand, bool p_can_assign) {
	push_error(R"("yield" was removed in Godot 4.0. Use "await" instead.)");
	return nullptr;
}
GDScriptParser::ExpressionNode *GDScriptParser::parse_invalid_token(ExpressionNode *p_previous_operand, bool p_can_assign) {