Do not crash when the scanned card is not associated with any account

[kjagiello]

Solves #28

[codecov]

Codecov Report

Merging #29 into develop will increase coverage by 1.48%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop      #29      +/-   ##
===========================================
+ Coverage    87.96%   89.44%   +1.48%     
===========================================
  Files           73       73              
  Lines         2500     2519      +19     
  Branches       134      135       +1     
===========================================
+ Hits          2199     2253      +54     
+ Misses         283      244      -39     
- Partials        18       22       +4

Impacted Files	Coverage Δ
src/foobar/views.py	94.11% <100%> (+7.45%)	✅
src/foobar/tests/test_views.py	100% <100%> (ø)	✅
src/shop/models.py	84.07% <0%> (ø)	✅
src/shop/admin.py	59.69% <0%> (+1.52%)	✅
src/wallet/admin.py	91.3% <0%> (+4.34%)	✅
src/foobar/admin.py	69.56% <0%> (+9.42%)	✅
src/utils/models.py	100% <0%> (+33.33%)	✅
src/utils/widgets.py	100% <0%> (+50%)	✅
src/utils/forms.py	100% <0%> (+50%)	✅

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