Fix array distinct rule for objects that have null/undefined items

[Melchyore]

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Hi!

I noticed that helpers.isDistinct() doesn't currently ignore null and undefined values when checking uniqueness for arrays of objects, even though the documentation states that they should be ignored.

For example:

vine
  .array(
    vine.object({
      sku: vine.string().minLength(1).maxLength(10),
    })
  )
  .distinct('sku')

with:

{
  "variants": [
    { "sku": null },
    { "sku": null }
  ]
}

currently fails with a "sku is not unique" error because null values are converted to the same key during the uniqueness check.

I've updated the implementation to skip uniqueness checks whenever one or more of the fields used to build the composite key are null or undefined, matching the documented behavior and the implementation for primitive arrays.

I've opened this PR to fix that inconsistency.

[thetutlage]

Looks good. Thanks!

Can you also update the docs to mention that not only the missing keys, the objects with undefined and null values are also skipped.

https://vinejs.dev/docs/types/array#treatment-of-objects-with-missing-keys

[Melchyore]

Sure thing!

#54