If null happens to coincide with one of the image widths or formats passed in, duplicate copies are generated

[AleksandrHovhannisyan]

Steps to reproduce:

1. Invoke the image plugin with the following source image (400px wide): https://images.unsplash.com/photo-1591160690555-5debfba289f0?ixid=MnwxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8&ixlib=rb-1.2.1&auto=format&fit=crop&w=400&q=80
2. Specify [null, 400] for the widths option.
3. Observe that the output JSON contains two 400-width images in the array.

This causes an issue on my site where an image is listed twice in the markup:

Also reproducible if:

1. You replace null with 400: [400, 400].
2. You pass in duplicate formats, like [null, 'png'] when the original format is png.

Expected behavior: The final widths/formats passed to the plugin should be unique. If a width/format happens to coincide with the source image width/format (null), only one copy should be returned.

Would be happy to contribute a PR (assuming this is in fact a bug and not the intended behavior)

[AleksandrHovhannisyan]

Bump, just wanna make sure this is at least on someone's radar 😄

[zachleat]

Appreciate this one, thank you!