Resolve to None when empty ID was passed

[maarcingebala]

This is a small improvement to the get_node_or_error function - if empty ID was passed as an argument it should immediately resolve to None. Without this change, multiple errors will be returned if empty String was passed as an ID to a mutation:

Currently, if we pass an empty string as an ID argument in a mutation, it will result in two errors:

{
    "field": "category",
    "message": "Couldn't resolve to a node: "
},
{
    "field": "category",
     "message": "This field cannot be null."
}

This PR changes this behavior to immediately return None in the get_node_or_error function so that the actual object resolver is not fired. This will result in only one meaningful error:

{
    "field": "category",
     "message": "This field cannot be null."
}

Edit: not related, but this PR also includes one minor TS types update, as it was out-of-date.

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 #3497 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3497      +/-   ##
==========================================
+ Coverage   89.89%   89.89%   +<.01%     
==========================================
  Files         241      241              
  Lines       13107    13109       +2     
  Branches     1322     1323       +1     
==========================================
+ Hits        11782    11784       +2     
  Misses        921      921              
  Partials      404      404

Impacted Files	Coverage Δ
saleor/graphql/core/mutations.py	92.75% <100%> (+0.07%)	⬆️

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