### 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
ForNode *parse_for();
	IfNode *parse_if(const String &p_token = "if");
	MatchNode *parse_match();
	MatchBranchNode *parse_match_branch();
	PatternNode *parse_match_pattern(PatternNode *p_root_pattern = nullptr);
	WhileNode *parse_while();
	
	ExpressionNode *parse_expression(bool p_can_assign, bool p_stop_on_assign = false);
	ExpressionNode *parse_precedence(Precedence p_precedence, bool p_can_assign, bool p_stop_on_assign = false);
	ExpressionNode *parse_literal(ExpressionNode *p_previous_operand, bool p_can_assign);
	LiteralNode *parse_literal();
	ExpressionNode *parse_self(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_identifier(ExpressionNode *p_previous_operand, bool p_can_assign);
	IdentifierNode *parse_identifier();
	ExpressionNode *parse_builtin_constant(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_unary_operator(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_binary_operator(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_binary_not_in_operator(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_ternary_operator(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_assignment(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_array(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_dictionary(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_call(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_get_node(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_preload(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_grouping(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_cast(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_await(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_attribute(ExpressionNode *p_previous_operand, bool p_can_assign);
	ExpressionNode *parse_subscript(ExpressionNode *p_previous_operand, bool p_can_assign);