Fix json game marshalling

[joshprzybyszewski]

What broke / What you're adding

dynamo exposed an issue in the game unmarshalling. The resulting Hands field is empty right after creation, and that panics when we try to deal out cards.

How you did it

write a unit test to verify that. then simply check for the nil fields and instantiate them to empty.

How to test it and how to try to break it

CI was broken. now CI is happy.

[codecov]

Codecov Report

Merging #104 (12cb8bc) into master (e5be80f) will decrease coverage by 0.01%.
The diff coverage is 66.67%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   69.79%   69.79%   -0.01%     
==========================================
  Files          80       80              
  Lines        3476     3482       +6     
==========================================
+ Hits         2426     2430       +4     
- Misses        831      832       +1     
- Partials      219      220       +1     

Impacted Files	Coverage Δ
jsonutils/gameUnmarshaller.go	57.89% <66.67%> (+4.05%)	⬆️
server/persistence/mongodb/mongodb_service_game.go	68.66% <0.00%> (-1.49%)	⬇️
server/play/cut.go	50.00% <0.00%> (+5.88%)	⬆️

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