### 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
consume(GDScriptTokenizer::Token::NEWLINE, R"(Expected a newline after "match" statement.)");
	if (!consume(GDScriptTokenizer::Token::INDENT, R"(Expected an indented block after "match" statement.)")) {
		complete_extents(match);
		return match;
	}
	bool all_have_return = true;
	bool have_wildcard = false;
	while (!check(GDScriptTokenizer::Token::DEDENT) && !is_at_end()) {
		MatchBranchNode *branch = parse_match_branch();
		if (branch == nullptr) {
			advance();
			continue;
		}
#ifdef DEBUG_ENABLED
		if (have_wildcard && !branch->patterns.is_empty()) {
			push_warning(branch->patterns[0], GDScriptWarning::UNREACHABLE_PATTERN);
		}
#endif
		have_wildcard = have_wildcard || branch->has_wildcard;
		all_have_return = all_have_return && branch->block->has_return;
		match->branches.push_back(branch);
	}
	complete_extents(match);
	consume(GDScriptTokenizer::Token::DEDENT, R"(Expected an indented block after "match" statement.)");
	if (all_have_return && have_wildcard) {
		current_suite->has_return = true;
	}
	return match;
}
GDScriptParser::MatchBranchNode *GDScriptParser::parse_match_branch() {