fix(gomock): display nil slices distinctly from empty slices in error messages

[wasim-builds]

What does this PR do?

Fixes confusing error messages when comparing nil slices with empty slices in gomock error output.

Problem

When a nil slice is passed where an empty slice is expected (or vice versa), the error message shows identical values for both "Got" and "Want", making it impossible to distinguish them:

Got: [] ([][32]uint8)
Want: is equal to [] ([][32]uint8)

This is because fmt.Sprintf("%v", ...) formats both nil slices and empty slices as [].

Solution

Update formatGottenArg to detect nil slices using reflect.ValueOf and display them as nil instead of []:

Got: nil ([][32]uint8)
Want: is equal to [] ([][32]uint8)

Changes

• gomock/call.go: Updated formatGottenArg to check if the argument is a nil slice using reflection, and format it as nil (<type>) instead of [] (<type>).
• gomock/controller_test.go: Added SliceOfArraysMethod to the test Subject struct. Added TestNilSliceVsEmptySlice and TestNilSliceOfArraysVsEmpty test cases to verify the fix for both []byte and [][32]byte types (the exact type from the original issue report).

Testing

All existing tests pass. Two new test cases added:

• TestNilSliceVsEmptySlice - verifies []byte nil vs empty distinction
• TestNilSliceOfArraysVsEmpty - verifies [][32]byte nil vs empty distinction (the exact scenario from issue Wrong print msg when comparing initialized and uninitialized slice #69)

Closes #69

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All committers have signed the CLA.

[wasim-builds]

By the way, I've really been enjoying contributing to this project! My goal is to get more involved in open source to help maintain projects and learn from great engineering teams. Is there a process or pathway for becoming a member of the uber-go GitHub organization? I'd love to continue helping out where I can!

[wasim-builds]

Hey 👋 Friendly bump on this one — fixes confusing gomock error messages when comparing nil vs empty slices. Small change, ready for review when you have a moment! 🙏