statement-distribution: Fix false warning (paritytech#4727)

... when backing group is of size 1.

Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>

polkadot/node/network/statement-distribution/src/v2/cluster.rs

@@ -429,7 +429,9 @@ impl ClusterTracker {
 
 	pub fn warn_if_too_many_pending_statements(&self, parent_hash: Hash) {
 		if self.pending.iter().filter(|pending| !pending.1.is_empty()).count() >=
-			self.validators.len()
+			self.validators.len() &&
+			// No reason to warn if we are the only node in the cluster.
+			self.validators.len() > 1
 		{
 			gum::warn!(
 				target: LOG_TARGET,