Fix integer underflow in try_range_response for empty files

[Darksonn]

Thanks to @mufeedvh for reporting this bug.

[seanmonstar]

Just to add a note from spec:

For a GET request, a valid bytes range-spec is satisfiable if it is either:

• an int-range with a first-pos that is less than the current length of the selected representation

So, with a file of 0 bytes, the first-pos of 0 is not less than the length, and thus is now rejected.

[yanns]

From CI:

error: the borrowed expression implements the required traits
   --> axum-extra/src/response/file_stream.rs:615:34
    |
615 |         std::fs::write(filename, &[]).unwrap();
    |                                  ^^^ help: change this to: `[]`

[Darksonn]

Fixed thanks.