Fix Hanyu Pinyin's empty rime handling #714

Job	Run time
Aarch64 cross-compile smoke test	21s
ubuntu-latest, preset=rust-release	1m 27s
Coverage preset=rust-coverage	1m 59s
ubuntu-latest, preset=rust-with-sqlite-release	2m 33s
Coverage preset=rust-with-sqlite-coverage	1m 46s
ubuntu-latest, preset=c99-release	51s
Coverage preset=c99-coverage	1m 9s
ubuntu-latest, preset=c99-with-uhash-release	42s
Coverage preset=c99-with-uhash-coverage	1m 7s
windows-latest, preset=rust-release	3m 0s
windows-latest, preset=rust-with-sqlite-release	2m 53s
windows-latest, preset=c99-release	2m 3s
windows-latest, preset=c99-with-uhash-release	1m 49s
macos-latest, preset=rust-release	1m 9s
macos-latest, preset=rust-with-sqlite-release	1m 4s
macos-latest, preset=c99-release	24s
macos-latest, preset=c99-with-uhash-release	23s
Final Results	1s
	24m 41s

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