Filter empty keys in 'tree' (folder nodes)

Fixes #106.

@sethvargo this should work for the 99% case, but what I realized in doing this is that we eliminate the possibility of getting the values of "folder" nodes in both `ls` and `tree`. I thought maybe we should add an extra condition, which would include the "folder" key if it had a value, but not if it didn't. This complicates things a little more, and I'm not sure we would ever need it, so it's not part of this PR. Just something to think about.