feat(reward): enable empty reward coins for MsgSetRewards

[Pantani]

What does this PR does?

Enable empty coins reward in MsgSetRewards so we can remove all rewards for a campaign

How to test?

• Create the chain and set reward

spnd tx profile create-coordinator --from alice && \
spnd tx launch create-chain orbit-1 orbit.com 0xaaa --from alice && \
spnd tx reward set-rewards 1 1000uspn 100000 --from alice

• Remove the reward

spnd tx reward set-rewards 1 0token 0 --from alice

[codecov]

Codecov Report

Merging #540 (dce8568) into develop (dfeb8aa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #540   +/-   ##
========================================
  Coverage    11.05%   11.06%           
========================================
  Files          234      234           
  Lines        54129    54131    +2     
========================================
+ Hits          5985     5987    +2     
+ Misses       47969    47968    -1     
- Partials       175      176    +1     

Impacted Files	Coverage Δ
x/reward/types/message_set_reward.go	29.62% <ø> (-7.04%)	⬇️
x/reward/keeper/msg_server_set_reward.go	84.61% <100.00%> (+1.05%)	⬆️

[aljo242]

lgtm!

[lumtis]

I think we can totally remove the reward pool from the store if rewards are set to 0

I think we can also have the same behavior when the height is set to 0

[lumtis]

CLI works well 👍