fix(api): do not include null values in ModelMutations.create

[ragingsquirrel3]

Addresses #2492

GraphQL model helpers have mutation helpers that generate JSON input for mutation variables based on models' toJson() methods. In the case of the model having a null value for an owner field, that null value gets passed to AppSync which will reject the mutation. This PR removes null fields from input variables when calling ModelMutations.create(). That avoids the error for the case of owner fields and generally simplifies client logic, allowing the server to set null values if omitted.

Note the original issue states "Model helpers should not pass null fields" but I believe that in general, there are valid cases of setting null values in mutations, like unsetting a nullable field that had previously been set to something non-null. For that reason, this PR only scopes the fix to create operations, which does not have any need to explicitly send null values.

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[codecov-commenter]

Codecov Report

Merging #2504 (88ed7da) into next (7250c1c) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             next    #2504      +/-   ##
==========================================
+ Coverage   56.19%   56.21%   +0.02%     
==========================================
  Files         115      115              
  Lines        6987     6991       +4     
==========================================
+ Hits         3926     3930       +4     
  Misses       3061     3061              

Flag	Coverage Δ
android-unit-tests	∅ <ø> (∅)
flutter-unit-tests	47.41% <100.00%> (+0.03%)	⬆️
ios-unit-tests	95.32% <ø> (ø)

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Impacted Files	Coverage Δ
...src/graphql/factories/graphql_request_factory.dart	95.45% <100.00%> (+0.14%)	⬆️
...src/graphql/factories/model_mutations_factory.dart	100.00% <100.00%> (ø)

[dnys1]

What about update? I'm guessing that would suffer the same problem.

You're correct that we should be setting null in cases where the user passed null but it's possible to distinguish these cases precisely. In the variables map, the presence of that field's key indicates the user passed a value for it. You need to check that instead of the value.

final variables = {
  'setNull': null,
};

// Doing this gives no insight about the user's intentions.
print(variables['setNull'] == null); // true
print(variables['doNotSetNull'] == null); // true

// Here we see what their intentions were.
print(variables.containsKey('setNull')); // true
print(variables.containsKey('doNotSetNull')); // false