rune: Simplify the empty sentinel value (#881) #1844

Job	Run time
nightly	4m 46s
msrv	1m 29s
docs	3m 43s
clippy	2m 9s
rustfmt	12s
basics	1s
test_lib	4m 20s
no_std (ubuntu-latest, minimal)	1m 17s
test_miri (rune)	3m 45s
no_std (macos-latest, minimal)	1m 31s
build_feature (alloc)	3m 26s
test_miri (rune-core)	56s
build_feature (cli)	4m 9s
test_miri (rune-alloc)	6m 10s
wasm	1m 7s
build_feature (cli,doc)	3m 59s
test_doc	6m 21s
build_feature (cli,fmt)	4m 6s
test_rune	3m 13s
build_feature (doc)	3m 38s
build_feature (workspace)	3m 37s
build_feature (languageserver)	4m 7s
build_feature (byte-code)	3m 22s
build_feature (capture-io)	3m 29s
build_feature (emit)	3m 30s
	1h 18m 23s

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