zfs_main: fix `zfs userspace` squashing unresolved entries

zfs_main: fix `zfs userspace` squashing unresolved entries

The `zfs userspace` squashes all entries with unresolved numeric
values into a single output entry due to the comparsion always
made by the string name which is empty in case of unresolved IDs.

Fix this by falling to a numerical comparison when either one
of string values is not found. This then compares any numerical
values after all with a name resolved.

Signed-off-by: Pavel Boldin <boldin.pavel@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4440