Fix decimal value argument in GraphQL

[fowczarek]

I want to merge this change because resolve #3428

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.

[maarcingebala]

We could also add a test for both cases:

• passing decimal directly to query
• passing it through variables

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3e6799e). Click here to learn what that means.
The diff coverage is 20%.

@@            Coverage Diff            @@
##             master    #3457   +/-   ##
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  Coverage          ?   89.73%           
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  Files             ?      237           
  Lines             ?    12943           
  Branches          ?     1312           
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  Hits              ?    11615           
  Misses            ?      924           
  Partials          ?      404

Impacted Files	Coverage Δ
saleor/graphql/core/types/common.py	89.47% <20%> (ø)

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