Fix arity error message with optional args. #5787

Job	Run time
build-rewatch (macos-13, x86_64-apple-darwin)	17s
build-compiler (ubuntu-24.04, ocaml-variants.5.2.1+options,ocaml-option-static, true, true)	1m 52s
build-rewatch (macos-14, aarch64-apple-darwin)	12s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.2.1+options,ocaml-option-static, true, true, t...	2m 18s
build-rewatch (ubuntu-24.04, x86_64-unknown-linux-musl)	10s
build-compiler (macos-13, 5.2.1, true)	2m 11s
build-rewatch (ubuntu-24.04-arm, aarch64-unknown-linux-musl)	11s
build-compiler (macos-14, 5.2.1, true)	2m 15s
build-rewatch (windows-latest, x86_64-pc-windows-gnu)	23s
build-compiler (windows-latest, 5.2.1, true)	2m 7s
build-compiler (ubuntu-24.04, ocaml-variants.5.0.0+options,ocaml-option-static)	1m 51s
build-compiler (ubuntu-24.04, ocaml-variants.4.14.2+options,ocaml-option-static)	1m 54s
build-compiler (ubuntu-24.04, ocaml-variants.4.13.0+options,ocaml-option-static)	1m 46s
package	1s
installationTest	1s
publish	1s
	17m 30s

Provide feedback