IndexSettings should not be Null in Mapper.BuildContext

Remove Nullable notation
Add unit test

Closes #20174

core/src/main/java/org/elasticsearch/index/mapper/Mapper.java

@@ -39,6 +39,7 @@ public abstract class Mapper implements ToXContent, Iterable<Mapper> {
         private final ContentPath contentPath;
 
         public BuilderContext(Settings indexSettings, ContentPath contentPath) {
+            assert indexSettings != null;
             this.contentPath = contentPath;
             this.indexSettings = indexSettings;
         }
@@ -47,16 +48,11 @@ public abstract class Mapper implements ToXContent, Iterable<Mapper> {
             return this.contentPath;
         }
 
-        @Nullable
         public Settings indexSettings() {
             return this.indexSettings;
         }
 
-        @Nullable
         public Version indexCreatedVersion() {
-            if (indexSettings == null) {
-                return null;
-            }
             return Version.indexCreated(indexSettings);
         }
     }

core/src/test/java/org/elasticsearch/index/mapper/MapperTests.java

@@ -0,0 +1,44 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.index.mapper;
+
+import org.elasticsearch.Version;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.equalTo;
+
+public class MapperTests extends ESTestCase {
+
+    public void testBuilderContextWithIndexSettings() {
+        Settings settings = Settings.builder().put(IndexMetaData.SETTING_VERSION_CREATED, Version.CURRENT).build();
+        Mapper.BuilderContext context = new Mapper.BuilderContext(settings, new ContentPath(1));
+
+        assertNotNull(context.indexSettings());
+        assertThat(context.indexSettings(), equalTo(settings));
+    }
+
+    public void testBuilderContextWithIndexSettingsAsNull() {
+        AssertionError e = expectThrows(AssertionError.class, () -> new Mapper.BuilderContext(null, new ContentPath(1)));
+    }
+
+
+}