Minor regression with null `checkOnSave/target` value

[iredelmeier]

rust-analyzer version: rust-analyzer 0.3.1293-standalone (2656297 2022-11-20)

rustc version: rustc 1.65.0 (897e37553 2022-11-02)

relevant settings: Reproduced in vim using with a brand new cargo new crate and coc-rust-analyzer < v0.69.6. I believe that v0.69.5 has the issue but that v0.69.6 handles it via this schema change.

#13290 changes the type of checkOnSave/target from an Option<String> to Vec<String>, and so any clients still setting it as null or failing to set it will run into problems. Setting it explicitly or defaulting to [] fixes it.

Not sure how much you care about config schema forwards compatibility. Apologies in advance if the expectation is that plugins are responsible for keeping up with breaking changes!