### 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
Error err = OK;
	GDScriptCodeGenerator *gen = codegen.generator;
	gen->clean_temporaries();
	codegen.start_block();
	if (p_add_locals) {
		_add_locals_in_block(codegen, p_block);
	}
	for (int i = 0; i < p_block->statements.size(); i++) {
		const GDScriptParser::Node *s = p_block->statements[i];
#ifdef DEBUG_ENABLED
		
		gen->write_newline(s->start_line);
#endif
		switch (s->type) {
			case GDScriptParser::Node::MATCH: {
				const GDScriptParser::MatchNode *match = static_cast<const GDScriptParser::MatchNode *>(s);
				codegen.start_block();
				
				GDScriptCodeGenerator::Address value = codegen.add_local("@match_value", _gdtype_from_datatype(match->test->get_datatype(), codegen.script));
				GDScriptCodeGenerator::Address value_expr = _parse_expression(codegen, err, match->test);
				if (err) {
					return err;
				}
				
				
				gen->write_assign(value, value_expr);
				if (value_expr.mode == GDScriptCodeGenerator::Address::TEMPORARY) {
					codegen.generator->pop_temporary();