Necessity of ? operator is not clear in List testcase example

This example is introducing the ? operator. Its use in the example does not appear necessary. The example compiles and runs even without using ?. It will also work correctly with an empty list, as illustrated below.

impl fmt::Display for List {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Extract the value using tuple indexing,
        // and create a reference to `vec`.
        let vec = &self.0;

        write!(f, "[");

        // Iterate over `v` in `vec` while enumerating the iteration
        // count in `count`.
        for (count, v) in vec.iter().enumerate() {
            // For every element except the first, add a comma.
            // Use the ? operator to return on errors.
            if count != 0 { write!(f, ", "); }
            write!(f, "{}", v);
        }

        // Close the opened bracket and return a fmt::Result value.
        write!(f, "]")
    }
}

fn main() {
    let v = List(vec![]);
    println!("{}", v);
}

Outputs
[]
as expected.

Is introducing ? operator in this example appropriate?

Read src/hello/print/print_display/testcase_list.md and compare the example with and without the ? operator, including the behavior of an empty list. Check the surrounding explanation of fmt::Result and error propagation, then update the example or its text so the operator's necessity and purpose are clear.