Don't provide `add_missing_match_arms` assist when upmapping match arm list failed
Fixes#15310
We shouldn't provide the assist when we fail to find the original match arm list.
Note that this PR will temporarily make the assist not applicable when attribute macro operates on the match expression in question, just like the case in #15310, for most of the current stable toolchain users. This is because the sysroot-abi proc-macro-srv on the current stable [discards] spans for `Group` delimiters in some code paths, which the popular `proc-macro2` crate almost always calls, and it makes the identity of match arm list's brackets lost, leading to the upmapping failure. This has been fixed by #14960, which will land in the next stable, 1.71.
[discards]: 8ede3aae28/src/tools/rust-analyzer/crates/proc-macro-srv/src/abis/abi_sysroot/ra_server.rs (L231)
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