It might be difficult for foreigners to differentiate word combinations, even native speakers are.
心虛 is a VH, an intransitive state verb. Specifically, it is a subject(心)-predicate(虛) construction.
When your heart feels rather empty, you feel diffident.
There is a similar phrase 膽/dan3/: gut 虛. When a man is not with plenty of guts, he is timid or frightened.
Though 虛心 is a VH, it's a verb(虛 to reserve)-obj(心) construction.
When you reserve your heart, you won't be occupated with bias.
There is a similar construction 虛左, describing to reserve an honored post for someone competent.