### 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
push_error(vformat(R"(Parameter with name "%s" was already declared for this %s.)", parameter->identifier->name, p_type));
			} else {
				p_function->parameters_indices[parameter->identifier->name] = p_function->parameters.size();
				p_function->parameters.push_back(parameter);
				p_body->add_local(parameter, current_function);
			}
		} while (match(GDScriptTokenizer::Token::COMMA));
	}
	pop_multiline();
	consume(GDScriptTokenizer::Token::PARENTHESIS_CLOSE, vformat(R"*(Expected closing ")" after %s parameters.)*", p_type));
	if (match(GDScriptTokenizer::Token::FORWARD_ARROW)) {
		make_completion_context(COMPLETION_TYPE_NAME_OR_VOID, p_function);
		p_function->return_type = parse_type(true);
		if (p_function->return_type == nullptr) {
			push_error(R"(Expected return type or "void" after "->".)");
		}
	}
	if (!p_function->source_lambda && p_function->identifier && p_function->identifier->name == GDScriptLanguage::get_singleton()->strings._static_init) {
		if (!p_function->is_static) {
			push_error(R"(Static constructor must be declared static.)");
		}
		if (p_function->parameters.size() != 0) {
			push_error(R"(Static constructor cannot have parameters.)");
		}
		current_class->has_static_data = true;
	}
	
	consume(GDScriptTokenizer::Token::COLON, vformat(R"(Expected ":" after %s declaration.)", p_type));
}
GDScriptParser::FunctionNode *GDScriptParser::parse_function(bool p_is_static) {