rpk: add nil check for maintenance mode

To avoid a segfault, we must check if
b.Maintenance is not nil because a user might try
to decommission a node with an older version that
doesn't have the maintenance mode feature (<v22).

src/go/rpk/pkg/cli/cmd/redpanda/admin/brokers/brokers.go

@@ -108,7 +108,9 @@ leader handles the request.
 			b, err := cl.Broker(cmd.Context(), broker)
 			out.MaybeDie(err, "unable to initialize admin client: %v", err)
 
-			if b.Maintenance.Draining {
+			// Old brokers (< v22.1) don't have maintenance mode, so we must
+			// check if b.Maintenance is not nil.
+			if b.Maintenance != nil && b.Maintenance.Draining {
 				out.Die(`Node cannot be decommissioned while it is in maintenance mode.
 Take the node out of maintenance mode first by running: 
     rpk cluster maintenance disable %v`, broker)