Only show suggested code when it is non-empty

[camelid]

This is to match rustc's behavior, and to avoid weird diagnostic boxes that look like this:

unnecessary trailing semicolons

note: `#[warn(redundant_semicolons)]` on by default
help: remove these semicolons: ``

After this change, the last line should look like this:

help: remove these semicolons

[Xanewok]

Looks good, thanks!