Why doesn't 虛 contradict 謙虛?
虛 = empty, the opposite of 虛(empty) is 實 (solid)
虛 in 謙虛 refers to 虛心 (empty heart = open mind)
A cup full of water cannot take in new water
A mind full of preconceptions cannot take in new ideas
謙虛 = modest, humble, and open-minded (willing to listen and learn)
Also, look at my answer to this question
What does 虚怀若谷 mean?