Fix two bugs for blank voice (empty speech): (#1403)

jianganghan

2024-02-29 f272eb4ef78e12a1ef4ae451b114f980a2682d51

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p m    p.m.
pm    p.m.
p.m.
p.m    p.m.
am    a.m.
a.m.
a.m    a.m.
a m    a.m.