rune: Simplify the empty sentinel value #1841

Job	Run time
nightly	3m 14s
msrv	1m 31s
docs	3m 0s
clippy	2m 14s
rustfmt	13s
basics	1s
test_doc	6m 43s
test_lib	4m 27s
no_std (ubuntu-latest, minimal)	1m 11s
build_feature (alloc)	3m 30s
no_std (macos-latest, minimal)	1m 26s
build_feature (cli)	4m 6s
test_miri (rune)	3m 36s
test_rune	3m 8s
build_feature (cli,doc)	4m 4s
test_miri (rune-core)	41s
wasm	1m 7s
build_feature (cli,fmt)	3m 56s
test_miri (rune-alloc)	6m 13s
build_feature (doc)	3m 38s
build_feature (workspace)	3m 31s
build_feature (languageserver)	3m 58s
build_feature (byte-code)	3m 27s
build_feature (capture-io)	3m 27s
build_feature (emit)	3m 29s
	1h 15m 51s

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