This element was previously always put in the generated element.
Since Rust 1.32.0 (so basically forever ago) we can use the $(…)? construct in macros to mean one or zero times this chunk of tokens. This allows making the last comma optional in lists of things.
This doesn’t change anything to the name of the crate, just makes autocompletion easier by not sharing the same prefix as the xmpp crate.
