Encoder needs to support nullable return types for relationship self/related links

[lindyhopchris]

The underlying neomerx encoder doesn't at the moment support nullable return types for the relationship self/related links. This results in errors (reported on multiple issues) when encoding a relationship response if the developer has disabled relationship links on their resource's relationship.

This really is a bug in the encoder: the spec doesn't say the relationship must have links, so the encoder should support nullable return types.

As we're now using a fork of the neomerx encoder, we should be able to fix this bug on our fork. It's looking like neomerx has disappeared - but if they ever return, we can submit a PR on the upstream repo to fix it there too.

[lindyhopchris]

Have managed to fix this without needing to make any changes to the Neomerx encoder. Previously the encoder-neomerx package was assuming that both the self and the related links always existed on the relationship, and therefore called the Neomerx methods to add both to the document. This was incorrect, as one or both might not exist.

Now we get the links from the relationship, if they are not empty just use the Neomerx encoder's withLinks() method to merge them in. The added benefit of this is that if there are any non-standard links on the relationship, these will now be merged too.

I've also safely handled getting the relationship from the resource. While imho there should always be a relationship, some developers have been hiding the relationship on the resource which means it isn't available to the encoder to get the relationship links. Previously this was causing an exception to be thrown. I've adjusted the code to handle there not being a relationship so it doesn't fail in this scenario. As it's only trying to merge the links, it should interpret a missing relation as no links rather than causing an error.

[lindyhopchris]

Closing as will be in the 1.1 release.