### 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
if (err) {
					_set_error(vformat(R"(Could not compile base class "%s" from "%s": %s)", base->fully_qualified_name, base->path, error_names[err]), nullptr);
					return err;
				}
				if (base_root.is_valid()) {
					base = Ref<GDScript>(base_root->find_class(base->fully_qualified_name));
				}
				if (base.is_null()) {
					_set_error(vformat(R"(Could not find class "%s" in "%s".)", base->fully_qualified_name, base->path), nullptr);
					return ERR_COMPILATION_FAILED;
				}
				ERR_FAIL_COND_V(!base->is_valid() && !base->reloading, ERR_BUG);
			}
			p_script->base = base;
			p_script->_base = base.ptr();
			p_script->member_indices = base->member_indices;
		} break;
		default: {
			_set_error("Parser bug: invalid inheritance.", nullptr);
			return ERR_BUG;
		} break;
	}
	for (int i = 0; i < p_class->members.size(); i++) {
		const GDScriptParser::ClassNode::Member &member = p_class->members[i];
		switch (member.type) {
			case GDScriptParser::ClassNode::Member::VARIABLE: {
				const GDScriptParser::VariableNode *variable = member.variable;
				StringName name = variable->identifier->name;
				GDScript::MemberInfo minfo;
				minfo.index = p_script->member_indices.size();