Unique validation SKU when default variant create in GraphQL API

[fowczarek]

I want to merge this change because resolve #3543

Screenshots

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 #3555 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3555      +/-   ##
==========================================
+ Coverage   89.83%   89.84%   +<.01%     
==========================================
  Files         250      250              
  Lines       13207    13213       +6     
  Branches     1334     1337       +3     
==========================================
+ Hits        11865    11871       +6     
  Misses        930      930              
  Partials      412      412

Impacted Files	Coverage Δ
saleor/graphql/product/mutations/products.py	95.56% <100%> (+0.05%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9b48cc9...19c9f55. Read the comment docs.