Fix null value usage for index.number_of_routing_shards setting. (#76069)

Ensure that the same number of routing shards is used for new indices
when `index.number_of_routing_shards` is not specified and `null` as value is specified.

Without this change if no index.number_of_routing_shards has been specified then
logic inside the MetadataCreateIndexService#getIndexNumberOfRoutingShards(...) kicks and
calculates a default based on number of primary shards use that as the true number of routing shards.
If a value is specified (including null) then the value the true number of routing shards is
determined from the IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING setting and
in the case of when null is specified then the default value is gathered from
the IndexMetadata.INDEX_NUMBER_OF_SHARDS_SETTING setting.

Closes #75139

server/src/main/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexService.java

@@ -819,7 +819,9 @@ public class MetadataCreateIndexService {
             // in this case we either have no index to recover from or
             // we have a source index with 1 shard and without an explicit split factor
             // or one that is valid in that case we can split into whatever and auto-generate a new factor.
-            if (IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.exists(indexSettings)) {
+            // (Don't use IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.get(indexSettings) here, otherwise
+            // we get the default value when `null` has been provided as value)
+            if (indexSettings.get(IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey()) != null) {
                 routingNumShards = IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.get(indexSettings);
             } else {
                 routingNumShards = calculateNumRoutingShards(numTargetShards, indexVersionCreated);

server/src/test/java/org/elasticsearch/cluster/metadata/MetadataCreateIndexServiceTests.java

@@ -900,6 +900,29 @@ public class MetadataCreateIndexServiceTests extends ESTestCase {
         assertThat(targetRoutingNumberOfShards, is(9));
     }
 
+    public void testGetIndexNumberOfRoutingShardsNullVsNotDefined() {
+        int numberOfPrimaryShards = randomIntBetween(1, 16);
+        Settings indexSettings = settings(Version.CURRENT)
+            .put(INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey(), (String) null)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), numberOfPrimaryShards)
+            .build();
+        int targetRoutingNumberOfShardsWithNull = getIndexNumberOfRoutingShards(indexSettings, null);
+        indexSettings = settings(Version.CURRENT)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), numberOfPrimaryShards)
+            .build();
+        int targetRoutingNumberOfShardsWithNotDefined = getIndexNumberOfRoutingShards(indexSettings, null);
+        assertThat(targetRoutingNumberOfShardsWithNull, is(targetRoutingNumberOfShardsWithNotDefined));
+    }
+
+    public void testGetIndexNumberOfRoutingShardsNull() {
+        Settings indexSettings = settings(Version.CURRENT)
+            .put(INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey(), (String) null)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 2)
+            .build();
+        int targetRoutingNumberOfShardsWithNull = getIndexNumberOfRoutingShards(indexSettings, null);
+        assertThat(targetRoutingNumberOfShardsWithNull, is(1024));
+    }
+
     public void testGetIndexNumberOfRoutingShardsYieldsSourceNumberOfShards() {
         Settings indexSettings = Settings.builder()
             .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 3)