fix: Only set False to clear FileFields when updating an instance

[bellini666]

Fix #599

Summary by Sourcery

This pull request fixes a bug where FileField values were being incorrectly cleared during instance creation. It ensures that FileField values are only set to False to clear them when updating an existing instance. Additionally, a new test has been added to verify the correct handling of optional FileField values during creation.

• Bug Fixes:
  • Ensure FileField values are only set to False to clear them when updating an existing instance, preventing unintended clearing during creation.
• Tests:
  • Add a test to verify that optional FileField values are handled correctly when not set during creation.

[sourcery-ai]

Reviewer's Guide by Sourcery

This pull request addresses issue #599 by ensuring that FileFields are only cleared (set to False) when updating an existing instance and not during creation. A new test case was added to verify the correct behavior when an optional file field is not set during creation.

File-Level Changes

Files	Changes
tests/mutations/test_mutations.py strawberry_django/mutations/resolvers.py	Added a test case to cover the scenario of not setting an optional file field during creation and updated the resolver logic to only clear FileFields when updating an existing instance.

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[sourcery-ai]

Hey @bellini666 - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟡 General issues: 1 issue found
• 🟢 Security: all looks good
• 🟡 Testing: 3 issues found
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.73%. Comparing base (6dc6be9) to head (6499d95).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #601      +/-   ##
==========================================
- Coverage   88.84%   88.73%   -0.12%     
==========================================
  Files          41       41              
  Lines        3605     3605              
==========================================
- Hits         3203     3199       -4     
- Misses        402      406       +4     

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