Add test for #4804

Add a failing test for the associated loading failure.
Skip hydrating associations when they are empty.

When all columns are null/[] the related entity should be skipped. While
the null case was added in 34109118c7a43a221b099e0527588ea3d265b694, the
case for associated entities with deeper non-joined associations that
are also empty was missed. By treating [] as empty we cover that case as
well.

Fixes #4804
Merge pull request #4834 from cakephp/issue-4804

Fix deep associations being hydrated incorrectly.