Require non-empty message in orderAddNote mutation

[maarcingebala]

Fixes #5314

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. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #5316 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5316      +/-   ##
==========================================
+ Coverage   91.59%   91.59%   +<.01%     
==========================================
  Files         283      283              
  Lines       18154    18162       +8     
  Branches     1580     1581       +1     
==========================================
+ Hits        16628    16636       +8     
  Misses       1103     1103              
  Partials      423      423

Impacted Files	Coverage Δ
saleor/graphql/order/mutations/orders.py	96.65% <100%> (+0.1%)	⬆️

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[django-queries]

Here is the report for c039cf5 (mirumee/saleor @ fix-empty-order-note)
Base comparison is 3718a0b.

No differences were found. (click me)

# api.benchmark category
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  category view                              	         23	         23	              0

# 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                    	         10	         10	              0
  complete checkout                          	          8	          8	              0
  create checkout                            	          5	          5	              1

# api.benchmark collection
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  collection view                            	         21	         21	              0

# api.benchmark homepage
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  featured products list                     	         20	         20	              0
  retrieve main menu                         	          8	          8	              0
  retrieve product list                      	          6	          6	              0
  retrieve secondary menu                    	          8	          8	              0
  retrieve shop                              	          2	          2	              0
  user checkout details                      	         53	         53	             28

# api.benchmark product
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product details                            	         17	         17	              3
  retrieve product attributes                	          9	          9	              0

# api.benchmark variant
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  product variant bulk create                	         51	         51	              3
  retrieve variant list                      	         23	         23	              9

# api product sorting attributes
  test name                                  	left count 	right count	duplicate count
  -------------------------------------------	-----------	-----------	---------------
  sort product not having attribute data     	         21	         21	              0