Optionally don't report foo.length() > 0

[tmssngr]

Currently, it suggests to simplify it to !foo.is_empty(), but I find this negation harder to read. Suggestion: (optionally) only warn for the foo.length() == 0-case. I don't like the clippy-warning in Rust either.

[ivov]

Agree, for > 0 and != 0 "simpler" is overstated. Relaxed this advisory at #806.