fix(paperwallet): fix panic when asset is zero

[daydoing]

What have you changed and why?

In backtesting, a panic occurs when the strategy has no trades.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.05 ⚠️

Comparison is base (13d871e) 40.44% compared to head (782dbdd) 40.40%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
- Coverage   40.44%   40.40%   -0.05%     
==========================================
  Files          19       19              
  Lines        2893     2896       +3     
==========================================
  Hits         1170     1170              
- Misses       1647     1650       +3     
  Partials       76       76              

Impacted Files	Coverage Δ
exchange/paperwallet.go	68.59% <0.00%> (-0.40%)	⬇️

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[rodrigo-brito]

Hi @daydoing, thank you for the contribution!