Fix product listing not showing in the voucher form

[NyanKiyoshi]

Closes #4060.

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. The code is documented (docstrings, project documentation).
8. GraphQL schema and type definitions are up to date.
9. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #4062 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4062      +/-   ##
==========================================
- Coverage   91.32%   91.31%   -0.01%     
==========================================
  Files         275      274       -1     
  Lines       14958    14931      -27     
  Branches     1448     1453       +5     
==========================================
- Hits        13660    13634      -26     
  Misses        904      904              
+ Partials      394      393       -1

Impacted Files	Coverage Δ
saleor/dashboard/discount/forms.py	93.33% <100%> (+0.31%)	⬆️
saleor/payment/gateways/braintree/forms.py	94.73% <0%> (-0.27%)	⬇️
saleor/payment/gateways/stripe/forms.py	96.42% <0%> (-0.24%)	⬇️
saleor/payment/gateways/razorpay/__init__.py	98.48% <0%> (-0.03%)	⬇️
saleor/payment/gateways/stripe/__init__.py	98.94% <0%> (-0.02%)	⬇️
saleor/payment/gateways/braintree/__init__.py	100% <0%> (ø)	⬆️
saleor/payment/gateways/stripe/utils.py	100% <0%> (ø)	⬆️
saleor/payment/gateways/razorpay/forms.py	96.87% <0%> (ø)	⬆️
saleor/payment/gateways/dummy/__init__.py	100% <0%> (ø)	⬆️
saleor/payment/interface.py
... and 1 more

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