fix: prevent nil DecCoin creation when converting Coins to DecCoins

[yun-yeo]

Description

Closes: #12902

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• confirmed the correct type prefix in the PR title
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[codecov]

Codecov Report

Merging #12903 (761b27a) into main (4fe7797) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main   #12903      +/-   ##
==========================================
- Coverage   55.87%   55.87%   -0.01%     
==========================================
  Files         646      646              
  Lines       54895    54898       +3     
==========================================
+ Hits        30675    30676       +1     
  Misses      21762    21762              
- Partials     2458     2460       +2     

Impacted Files	Coverage Δ
types/dec_coin.go	98.41% <50.00%> (-0.79%)	⬇️
x/group/keeper/keeper.go	56.29% <0.00%> (+0.39%)	⬆️

[tac0turtle]

ack

[alexanderbez]

@YunSuk-Yeo, sorry do you mind addressing the merge conflict prior to merge?