Adapt to adding support to store protos in indexes

In Ieadcb3b097 support to store protos in indexes was added in gerrit
core. Adapt the code correspondingly. Similarly to Lucene backend, ES
currently doesn't support storing of protos in index backend.

Change-Id: Ifd91ed4ee5a4a36817a3ccb152dba9fb0e8d2fd6
diff --git a/src/main/java/com/google/gerrit/elasticsearch/ElasticStoredValue.java b/src/main/java/com/google/gerrit/elasticsearch/ElasticStoredValue.java
index a02a715..bd7c1c3 100644
--- a/src/main/java/com/google/gerrit/elasticsearch/ElasticStoredValue.java
+++ b/src/main/java/com/google/gerrit/elasticsearch/ElasticStoredValue.java
@@ -18,6 +18,7 @@
 
 import com.google.gerrit.index.StoredValue;
 import com.google.gson.JsonElement;
+import com.google.protobuf.MessageLite;
 import java.sql.Timestamp;
 import java.time.Instant;
 import java.time.format.DateTimeFormatter;
@@ -83,4 +84,16 @@
         .map(f -> AbstractElasticIndex.decodeBase64(f.getAsString()))
         .collect(toImmutableList());
   }
+
+  @Override
+  public MessageLite asProto() {
+    // Elasticsearch does not store protos
+    return null;
+  }
+
+  @Override
+  public Iterable<MessageLite> asProtos() {
+    // Elasticsearch does not store protos
+    return null;
+  }
 }