fix: gen_struct_equality_fn null pointer crash

[get200]

Fix issue #25237

[huly-for-github]

Connected to Huly®: V_0.6-24681

[get200]

// D:\\v\\v\\vlib\\json\\tests\\json_option_struct_test.v
import json

pub struct SomeStruct {
pub mut:
	test ?string
}

pub struct MyStruct {
pub mut:
	result ?SomeStruct
	id     string
}

fn test_main() {
	a := MyStruct{
		id:     'some id'
		result: SomeStruct{}
	}
	encoded_string := json.encode(a)
	assert encoded_string == '{"result":{},"id":"some id"}'
	test := json.decode(MyStruct, encoded_string)!
	dump(test)
	dump(a)
	dump(test == a)
	assert test == a
}

$ v run D:\v\v\vlib\json\tests\json_option_struct_test.v               
[D:\\v\\v\\vlib\\json\\tests\\json_option_struct_test.v:22] test: MyStruct{
    result: Option(none)
    id: 'some id'
}
[D:\\v\\v\\vlib\\json\\tests\\json_option_struct_test.v:23] a: MyStruct{
    result: Option(SomeStruct{
        test: Option(none)
    })
    id: 'some id'
}
[D:\\v\\v\\vlib\\json\\tests\\json_option_struct_test.v:24] test == a: true

@spytheman may be this is a bug?