User input always true using the .is_ok functions

How can I read from the console and avoid crashing if the user presses enter without entering anything?

let mut objectnumber = String::new();
println!("mennyi súlyt szeretnél megnézni? 3.=255 int");
if io::stdin().read_line(&mut objectnumber).is_ok() {
   println!("{}csdcds", objectnumber);
}

//panic!("{}",objectnumber.len());
if objectnumber.len() < 3 {
   let localrandnum: u8 = thread_rng().gen_range(0..=255);
   println!("miután nem adtál meg semmit sem ezért {localrandnum}.at írtam be neked");
   objectnumber = localrandnum.to_string();
}
let objectnumber: u8 = objectnumber.trim().parse::<u8>().expect("null");

This is working but I don't think it should be done like this in Rust. I don't know why is it ok even on empty line while the parse is talking about it being empty.

Solution

Fixes I made:

read_line returns a Result indicating whether or not there was an error reading from stdin. Usually this only happens in very specific circumstances, so let's just unwrap this for now.
objectnumber.len() gives you the length of the string, not its content. To check its content for < 3, we need to parse() it to a u8 first.
Don't expect the parse, because this one depends on user input. Do a proper error handling instead. This is the point that will tell you if the number is >255, <0 or something else entirely. Let's for now just default to '0', because it will then get caught by the < 3.
Don't convert the number you generated with the thread_rng to a String; instead, write it to objectnumber directly. There is no point in converting it to a String and back.

use std::io;

use rand::{thread_rng, Rng};

fn main() {
    let mut objectnumber = String::new();
    println!("mennyi súlyt szeretnél megnézni? 3.=255 int");
    io::stdin()
        .read_line(&mut objectnumber)
        .expect("Unable to read from stdin!");

    // Convert from String to u8
    let mut objectnumber: u8 = objectnumber.trim().parse::<u8>().unwrap_or(0);

    if objectnumber < 3 {
        objectnumber = thread_rng().gen_range(0..=255);
        println!("miután nem adtál meg semmit sem ezért {objectnumber}.at írtam be neked");
    }

    println!("objectnumber: {}", objectnumber);
}

mennyi súlyt szeretnél megnézni? 3.=255 int
33
objectnumber: 33

mennyi súlyt szeretnél megnézni? 3.=255 int
1
miután nem adtál meg semmit sem ezért 191.at írtam be neked
objectnumber: 191