formatAttributes is a reduce; the empty-style branch returned '' instead of
the accumulator, so every attribute emitted before an empty `style=""`
attribute was dropped on stringify (e.g. `<div class="x" style=""> -> <div>`).
Return `attrs` to omit the empty style while keeping the rest.
Closes#682
Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: wyuc <wang-yc24@mails.tsinghua.edu.cn>