### 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 (subscript->base->type == GDScriptParser::Node::SELF && codegen.script) {
					GDScriptParser::IdentifierNode *identifier = subscript->attribute;
					HashMap<StringName, GDScript::MemberInfo>::Iterator MI = codegen.script->member_indices.find(identifier->name);
#ifdef DEBUG_ENABLED
					if (MI && MI->value.getter == codegen.function_name) {
						String n = identifier->name;
						_set_error("Must use '" + n + "' instead of 'self." + n + "' in getter.", identifier);
						r_error = ERR_COMPILATION_FAILED;
						return GDScriptCodeGenerator::Address();
					}
#endif
					if (MI && MI->value.getter == "") {
						
						gen->pop_temporary();
						
						return GDScriptCodeGenerator::Address(GDScriptCodeGenerator::Address::MEMBER, MI->value.index, _gdtype_from_datatype(subscript->get_datatype(), codegen.script));
					}
				}
				name = subscript->attribute->name;
				named = true;
			} else {
				if (subscript->index->is_constant && subscript->index->reduced_value.get_type() == Variant::STRING_NAME) {
					
					name = subscript->index->reduced_value;
					named = true;
				} else {
					
					index = _parse_expression(codegen, r_error, subscript->index);
					if (r_error) {
						return GDScriptCodeGenerator::Address();