iterator: test that we can `advance_into` empty dirs
iterator: test that we can `advance_into` empty dirs

Prior iterator implementations returned `GIT_ENOTFOUND` when
trying to advance into empty directories.  Ensure that we no longer
do that and simply handle them gracefully.

joshtriplett/libgit2,leoyanggit/libgit2,Corillian/libgit2,Corillian/libgit2,stewid/libgit2,Corillian/libgit2,leoyanggit/libgit2,leoyanggit/libgit2,leoyanggit/libgit2,leoyanggit/libgit2,Corillian/libgit2,stewid/libgit2,stewid/libgit2,Corillian/libgit2,joshtriplett/libgit2,joshtriplett/libgit2,leoyanggit/libgit2,joshtriplett/libgit2,stewid/libgit2,stewid/libgit2,joshtriplett/libgit2,joshtriplett/libgit2,stewid/libgit2,Corillian/libgit2