How did 零 come to mean zero? I understand that 零 originally meant rain, however for some reason it eventually became fragments and then also zero. How did the meaning of this character shift to become fragments (rain drops, maybe?) as well as zero?
零 originally meant rain


Not just rain, but specifically gentle rain.


  《說文・雨部》：＂零，徐雨也。＂


Semantic extension:

Gentle rain > scattered/fragmentary > small/fractional > zero.