zfs get -p only outputs 3 columns if "clones" property is empty

get_clones_string currently returns an empty string for filesystem
snapshots which have no clones. This breaks parsable `zfs get` output as
only three columns are output, instead of 4.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Fiddaman <github@m.fiddaman.uk>
Co-authored-by: matt <matt@fiddaman.net>
Closes #11837

lib/libzfs/libzfs_dataset.c

@@ -32,6 +32,7 @@
  * Copyright 2017-2018 RackTop Systems.
  * Copyright (c) 2019 Datto Inc.
  * Copyright (c) 2019, loli10K <ezomori.nozomu@gmail.com>
+ * Copyright (c) 2021 Matt Fiddaman
  */
 
 #include <ctype.h>
@@ -2385,7 +2386,7 @@ get_clones_string(zfs_handle_t *zhp, char *propbuf, size_t proplen)
 	nvpair_t *pair;
 
 	value = zfs_get_clones_nvl(zhp);
-	if (value == NULL)
+	if (value == NULL || nvlist_empty(value))
 		return (-1);
 
 	propbuf[0] = '\0';