DRONE_PREV_COMMIT_SHA will be null when tag evnet trigger

Drone Support
1

DRONE_PREV_COMMIT_SHA will be null when tag event trigger, push event is normal

example:

tag event: https://gitlab.drone.wsl.fun/shynome/drone-test/15/3
push event - https://gitlab.drone.wsl.fun/shynome/drone-test/13/3