libzutil: zfs_isnumber(): return false if input empty

zpool list, which is the only user, would mistakenly try to parse the
empty string as the interval in this case:

$ zpool list "a"

cannot open 'a': no such pool

$ zpool list ""

interval cannot be zero

usage: <usage string follows>

which is now symmetric with zpool get:

  $ zpool list ""
  cannot open '': name must begin with a letter

Avoid breaking the "interval cannot be zero" string.
There simply isn't a need for this, and it's user-facing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11841
Closes #11843