Fix jsonapi prev and next keys for unavailable links

[kobusjoubert]

Hi @ddnexus, thank you so much for this awesome Gem 💎

I came across an issue when using the Jsonapi extra.

The Problem

I'm getting a expected :page >= 1; got 0 exception when following a prev or next link with no content.

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The Fix

According to the JSON:API spec, it specifies to omit the key or return null for unavailable links.

If you're okay with this, I've applied a small change to return null for unavailable links in this pull request.

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Let me know if you want me to change anything @ddnexus.

[ddnexus]

Hi @kobusjoubert . Thanks for the PR.

I am conflicted about using null values or omitting... I even researched it and... it looks everybody is conflicted about null or omit. It looks like the two options are equally... meh! 🙃

So... the option that you choose looks visually better in the code, so that seems to be compelling enough for me. 😁

Please, could you fix the tests. Thank you!

[kobusjoubert]

Hi @ddnexus, that's awesome! Thank you so much for looking at this pull request 🥳

I just added some tests, runs good on my side.

[ddnexus]

Thank you!