Numbat panics on the date time format string `%:`

[rben01]

This seems to be caused by jiff. Not sure if simply updating jiff to the latest version, v0.2.18, would fix this, as the file mentioned in the panic message doesn't even exist in the jiff GitHub repo anymore.

To reproduce:

  █▄░█ █░█ █▀▄▀█ █▄▄ ▄▀█ ▀█▀    Numbat 1.18.0
  █░▀█ █▄█ █░▀░█ █▄█ █▀█ ░█░    https://numbat.dev/

>>> format_datetime("%:", now())

thread 'main' (41394987) panicked at /Users/brew/Library/Caches/Homebrew/cargo_cache/registry/src/index.crates.io-1949cf8c6b5b557f/jiff-0.2.14/src/fmt/strtime/format.rs:170:9:
index out of bounds: the len is 0 but the index is 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The format string is invalid, but obviously this should not cause numbat to panic. This seems unique to this particular format string; other invalid format strings don't lead to a panic, suggesting that this is a formatting logic bug somewhere in jiff.

[eligibleauthor]

Hey @rben01 @sharkdp, I would like to further investigate on this issue.

[eligibleauthor]

In my system, for numbat 1.18.0, I see that jiff is just throwing a validation error.