Fix voucher limit value when checkbox unchecked

[benekex2]

I want to merge this change because...

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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.

[fowczarek]

If you want to clear usageLimit, use null instead of 0.

[fowczarek]

Now if a user clear input field (eg. Minimal order value, Limit of Uses) and save, the dashboard try to make mutation with "" as value for this field. This will cause an error. If a user clears any decimal input field, we should send null instead of "".

[django-queries]

Here is the report for bb91d50 (mirumee/saleor @ fix-voucher-usage-limit-check)
Base comparison is c218fb0.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  add billing address to checkout     	         34	         34	             20
  add shipping to checkout            	          7	          7	              0
  checkout payment charge             	         14	         14	              0
  complete checkout                   	          6	          6	              0
  create checkout                     	         43	         43	             21

# api.benchmark homepage
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve main menu                  	          5	          5	              0
  retrieve product list               	          4	          4	              0
  retrieve secondary menu             	          5	          5	              0
  retrieve shop                       	          2	          2	              0

# api.benchmark product
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  product details                     	         13	         13	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	          9	          9	              2

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c218fb0). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4456   +/-   ##
=========================================
  Coverage          ?   90.72%           
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  Files             ?      291           
  Lines             ?    16785           
  Branches          ?     1711           
=========================================
  Hits              ?    15229           
  Misses            ?     1056           
  Partials          ?      500

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Requested changes were applied.