Display menu item form errors

[dominik-zeglen]

I want to merge this change because it fixes error displaying and empty value handling in menu item dialog.
Fixes #4547

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.

[django-queries]

Here is the report for bd2b27e (mirumee/saleor @ fix/display-menu-errors)
Base comparison is cf43e0e.

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                     	         41	         41	             20

# 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

Merging #4551 into master will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #4551     +/-   ##
=========================================
- Coverage   90.83%   90.64%   -0.2%     
=========================================
  Files         297      297             
  Lines       17447    17437     -10     
  Branches     1735     1748     +13     
=========================================
- Hits        15848    15805     -43     
- Misses       1092     1121     +29     
- Partials      507      511      +4

Impacted Files	Coverage Δ
saleor/core/extensions/manager.py	77.08% <0%> (-12.5%)	⬇️
saleor/dashboard/menu/utils.py	90.76% <0%> (-9.24%)	⬇️
saleor/core/templatetags/taxed_prices.py	80.95% <0%> (-7.94%)	⬇️
saleor/checkout/utils.py	83.64% <0%> (-0.39%)	⬇️
saleor/graphql/menu/mutations.py	97.26% <0%> (-0.32%)	⬇️
saleor/checkout/views/__init__.py	85.82% <0%> (-0.32%)	⬇️
saleor/product/utils/availability.py	97.18% <0%> (-0.16%)	⬇️
saleor/core/middleware.py	76.05% <0%> (-0.14%)	⬇️
saleor/checkout/forms.py	94.38% <0%> (-0.1%)	⬇️
saleor/dashboard/order/forms.py	87.36% <0%> (-0.07%)	⬇️
... and 30 more

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[benekex2]

Unnecessary braces

[benekex2]

Unnecessary braces