rune: Simplify the empty sentinel value #1843

Job	Run time
clippy	2m 14s
nightly	4m 52s
docs	3m 15s
msrv	1m 38s
rustfmt	17s
basics	1s
wasm	1m 7s
no_std (ubuntu-latest, minimal)	1m 16s
no_std (macos-latest, minimal)	2m 4s
test_doc	6m 15s
build_feature (alloc)	3m 25s
test_lib	4m 23s
build_feature (cli)	4m 21s
test_miri (rune)	3m 37s
test_rune	3m 2s
build_feature (cli,doc)	4m 10s
test_miri (rune-core)	1m 2s
build_feature (cli,fmt)	4m 3s
test_miri (rune-alloc)	5m 54s
build_feature (doc)	3m 38s
build_feature (workspace)	3m 32s
build_feature (languageserver)	3m 58s
build_feature (byte-code)	3m 30s
build_feature (capture-io)	3m 29s
build_feature (emit)	3m 21s
	1h 18m 24s

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