### 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
r_error = ERR_COMPILATION_FAILED;
							return GDScriptCodeGenerator::Address();
						}
					} else {
						r_error = ERR_COMPILATION_FAILED;
						return GDScriptCodeGenerator::Address();
					}
				}
			}
			for (int i = 0; i < arguments.size(); i++) {
				if (arguments[i].mode == GDScriptCodeGenerator::Address::TEMPORARY) {
					gen->pop_temporary();
				}
			}
			return result;
		} break;
		case GDScriptParser::Node::GET_NODE: {
			const GDScriptParser::GetNodeNode *get_node = static_cast<const GDScriptParser::GetNodeNode *>(p_expression);
			Vector<GDScriptCodeGenerator::Address> args;
			args.push_back(codegen.add_constant(NodePath(get_node->full_path)));
			GDScriptCodeGenerator::Address result = codegen.add_temporary(_gdtype_from_datatype(get_node->get_datatype(), codegen.script));
			MethodBind *get_node_method = ClassDB::get_method("Node", "get_node");
			gen->write_call_ptrcall(result, GDScriptCodeGenerator::Address(GDScriptCodeGenerator::Address::SELF), get_node_method, args);
			return result;
		} break;
		case GDScriptParser::Node::PRELOAD: {
			const GDScriptParser::PreloadNode *preload = static_cast<const GDScriptParser::PreloadNode *>(p_expression);
			
			return codegen.add_constant(preload->resource);
		} break;