IGNITE-26580 Improve code after self-review

Author: sergey-chugunov-1985

modules/core/src/main/java/org/apache/ignite/cache/affinity/rendezvous/MdcAffinityBackupFilter.java

@@ -106,6 +106,10 @@ public MdcAffinityBackupFilter(int dcsNum, int backups) {
 
         boolean res = false;
         String candidateDcId = candidate.dataCenterId();
+
+        if (candidateDcId == null)
+            throw new IllegalStateException("Data center ID is not specified for the node: " + candidate);
+
         Integer candDcPartsCopies = partsDistrMap.get(candidateDcId);
 
         if (candDcPartsCopies == null || candDcPartsCopies == -1) {

modules/core/src/test/java/org/apache/ignite/cache/affinity/rendezvous/MdcAffinityBackupFilterSelfTest.java

@@ -33,8 +33,8 @@
 import org.junit.Test;
 
 /**
- * Verifies behaviour of {@link MdcAffinityBackupFilter} - guarantees that each DC has at least one copy of every partition,
- * distribution uniformity.
+ * Verifies behaviour of {@link MdcAffinityBackupFilter} - guarantees that each DC has at least one copy of every partition.
+ * Verified distribution uniformity in each DC separately.
  */
 public class MdcAffinityBackupFilterSelfTest extends GridCommonAbstractTest {
     /** */