fix(router-macro): omit ? and stray & when query arguments are empty

[nicoburns]

Summary

Fixes #5792. Closes #5839 (supersedes it with an allocation-free approach).

Routable's generated Display impl unconditionally wrote ? before the query segment, so a route like /doc/:id?:folder with folder: None rendered as /doc/abc?. It also wrote & after every non-empty, non-final argument regardless of what followed, so /r?:a&:b with a: Some(1), b: None rendered as /r?a=1&.

The generated code now tracks a single bool and emits the separator lazily, only in front of arguments that actually produced output:

let mut wrote_query_argument = false;
// per `:arg`
let as_string = DisplayQueryArgument::new("arg", arg).to_string(); // pre-existing alloc
if !as_string.is_empty() {
    f.write_str(if wrote_query_argument { "&" } else { "?" })?;
    write!(f, "{}", utf8_percent_encode(&as_string, QUERY_ASCII_SET))?;
    wrote_query_argument = true;
}

The ?:..params (FullQuerySegment) path gets the same is_empty() guard. No new allocations; the trailing parameter on QueryArgument::write is gone since position no longer matters.

Test covers the full-query path, plus a two-argument route with every combination of present/absent arguments, round-tripping through from_str.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/2a0f4275713d441fb98a3b8b8d7f63a4
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/2a0f4275713d441fb98a3b8b8d7f63a4?variant=devin-insiders
Requested by: @nicoburns

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