### 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
struct ChainInfo {
					bool is_named = false;
					GDScriptCodeGenerator::Address base;
					GDScriptCodeGenerator::Address key;
					StringName name;
				};
				List<ChainInfo> set_chain;
				for (List<const GDScriptParser::SubscriptNode *>::Element *E = chain.back(); E; E = E->prev()) {
					if (E == chain.front()) {
						
						break;
					}
					const GDScriptParser::SubscriptNode *subscript_elem = E->get();
					GDScriptCodeGenerator::Address value = codegen.add_temporary(_gdtype_from_datatype(subscript_elem->get_datatype(), codegen.script));
					GDScriptCodeGenerator::Address key;
					StringName name;
					if (subscript_elem->is_attribute) {
						name = subscript_elem->attribute->name;
						gen->write_get_named(value, name, prev_base);
					} else {
						key = _parse_expression(codegen, r_error, subscript_elem->index);
						if (r_error) {
							return GDScriptCodeGenerator::Address();
						}
						gen->write_get(value, key, prev_base);
					}
					
					set_chain.push_front({ subscript_elem->is_attribute, prev_base, key, name }); 
					prev_base = value;
				}