IGNITE-13770 Fix NPE in Ignite.dataRegionMetrics with empty persistent region - Fixes #8506.

Signed-off-by: Ilya Kasnacheev <ilya.kasnacheev@gmail.com>

Author: alamar

modules/core/src/main/java/org/apache/ignite/internal/processors/cache/persistence/pagemem/PageMemoryImpl.java

@@ -1829,14 +1829,14 @@ public int activePagesCount() {
 
     /** {@inheritDoc} */
     @Override public int checkpointBufferPagesCount() {
-        return checkpointPool.size();
+        return checkpointPool == null ? 0 : checkpointPool.size();
     }
 
     /**
      * Number of used pages in checkpoint buffer.
      */
     public int checkpointBufferPagesSize() {
-        return checkpointPool.pages();
+        return checkpointPool == null ? 0 : checkpointPool.pages();
     }
 
     /**

modules/core/src/test/java/org/apache/ignite/internal/processors/cache/persistence/db/IgnitePdsDataRegionMetricsTest.java

@@ -89,6 +89,13 @@ public class IgnitePdsDataRegionMetricsTest extends GridCommonAbstractTest {
                     .setMaxSize(MAX_REGION_SIZE)
                     .setPersistenceEnabled(true)
                     .setMetricsEnabled(true))
+            .setDataRegionConfigurations(
+                new DataRegionConfiguration()
+                    .setName("EmptyRegion")
+                    .setInitialSize(INIT_REGION_SIZE)
+                    .setMaxSize(MAX_REGION_SIZE)
+                    .setPersistenceEnabled(true)
+                    .setMetricsEnabled(true))
             .setCheckpointFrequency(1000);
 
         cfg.setDataStorageConfiguration(memCfg);