Normalize missing entry case

[breathx]

Resolves #640.

@gear-tech/dev

Release Notes: If some of entry point (init, handle or handle_reply) isn't provided, it means now that this function is an empty function.

This code ⬇️

#![no_std]

use gstd::msg;

#[no_mangle]
pub unsafe extern "C" fn handle() {
    msg::reply_bytes("Handle", 0, 0);
}

now equals this one ⬇️

#![no_std]

use gstd::msg;

#[no_mangle]
pub unsafe extern "C" fn init() { }

#[no_mangle]
pub unsafe extern "C" fn handle() {
    msg::reply_bytes("Handle", 0, 0);
}

#[no_mangle]
pub unsafe extern "C" fn handle_reply() { }

It used to cause panic.