While it is not allowed for a tree to have an empty tree as child (e.g.
an empty directory), libgit2's tree builder makes it easy to create such
trees. As a result, some applications may inadvertently end up with such
an invalid tree, and we should try our best and handle them.

One such case is when diffing two trees, where one of both trees has
such an empty subtree. It was reported that this will cause our diff
code to fail. While I wasn't able to reproduce this error, let's still
add a test that verifies we continue to handle them correctly.