fix(test): DS transformer V2 tests should be update mutation on existing model

[lawmicha]

Issue #

Description

While going over previous integration tests, specifically DataStoreTransformerV2 tests. One of the tests was failing because we were trying to send nil values of the foreign key for create mutations. We shouldn't be sending a create mutation, but rather an update mutation on an existing model. updateMutations will set nil values for the FK to act as an removal of an assocation. The implementation change was introduced in #2701 to data-dev-preview and this test was not run to catch the failure

General Checklist

• Added new tests to cover change, if needed
• Build succeeds with all target using Swift Package Manager
• All unit tests pass
• All integration tests pass
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Documentation update for the change if required
• PR title conforms to conventional commit style
• New or updated tests include Given When Then inline code documentation and are named accordingly testThing_condition_expectation()
• If breaking change, documentation/changelog update with migration instructions

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