Ensured that no fields are returned when sparse fields is set to an empty value

[sliverc]

Fixes #1238

Description of the Change

When a sparse field is set to an empty value no fields (attributes/relationships) should be rendered whereas currently it rendered all fields.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[n2ygk]

I think the current behavior is supported by the spec:

If a client does not specify the set of fields for a given resource type, the server MAY send all fields, a subset of fields, or no fields for that resource type.

https://jsonapi.org/format/
8.4 Sparse Fieldsets

It could break a lot of implementations. Perhaps add a setting.

[sliverc]

I agree that the sentence you mentioned is a bit ambiguous, however it would not make any sense to first specify that The value of any fields[TYPE] parameter MUST and then afterward say that the server MAY. So the sentence you mention is, as far as I see it, if there is no query parameter set at all for a given resource type, but not if it is explicitly set to an empty value.

In any case, I have double-checked what was the behavior before change #1221. It turned out when defining an empty value it would also return no fields. So this means it is a regression and this PR will revert to the behavior we had before version 7.0.0. I have clarified this in the changelog as well now.

[sliverc]

Thanks @n2ygk for merging. Let me prepare a release to get those regressions out of the way.