Mark diffusion browse result as empty directory when appropriate

Summary:
When selecting children of a directory, it is possible that none of its
children exist anymore even though the directory still exists. After fetching
the children but before returning them, we should check whether there are any,
and if there are no children, set the reason as empty directory.

Test Plan:
In sandbox, browsed in diffusion to a directory that exists but has no
files and saw that it has a useful message instead of a vague exception.

Reviewers: epriestley, tuomaspelkonen, jungejason

Reviewed By: tuomaspelkonen

CC: aran, tuomaspelkonen

Differential Revision: 846