Fix 5288: Doesn't POST if attestations is empty. (#5318)

* changed to is_empty() and removed WARN

* added log argument

* fix: issue 5288

validator_client/src/attestation_service.rs

@@ -430,6 +430,11 @@ impl<T: SlotClock + 'static, E: EthSpec> AttestationService<T, E> {
             .flatten()
             .unzip();
 
+        if attestations.is_empty() {
+            warn!(log, "No attestations were published");
+            return Ok(None);
+        }
+
         // Post the attestations to the BN.
         match self
             .beacon_nodes