Azure metadata API may return empty zone sometimes. If that happens
shard-0 gets empty string as its rack, but propagates UNKNOWN_RACK to
other shards.
Empty zones response should be handled regardless.
refs: #12185
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#12274
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>