chore: allow empty mapping string

[lukebond]

it should be possible to specify an empty string as a mapping, e.g. let's say you want to copy some subtree of an object into the root of another, or vise versa. the to and from mapping fields are both required so we must support an empty string to indicate this case. as the code was before it would become the jsonpath $. because we were simply checking if the option was Some(String).

[lukebond]

i'm not happy with the Rust here. rust playground says this should work, but it doesn't: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021 so we have the version i've put in the PR. it works.

update: ah, it works in the playground because in that example it's a &str not a String. i am rusty indeed. any improvements welcome or we can merge like this.

[lukebond]

an alternative solution is to make those fields options in the crd, so the user just leaves one or the other out to specify the root. unsure if that's more or less intuitive