Don't pull translog from shadow engine

ShadowEngine doesn't have a translog but instead throws an
UOE when it's requested. ShadowIndexShard should not try to pull
stats for the translog either and should return null instead.

Closes #12730

core/src/main/java/org/elasticsearch/index/shard/ShadowIndexShard.java

@@ -26,6 +26,7 @@ import org.elasticsearch.index.engine.EngineConfig;
 import org.elasticsearch.index.merge.MergeStats;
 import org.elasticsearch.index.settings.IndexSettings;
 import org.elasticsearch.index.store.Store;
+import org.elasticsearch.index.translog.TranslogStats;
 
 import java.io.IOException;
 
@@ -82,4 +83,9 @@ public final class ShadowIndexShard extends IndexShard {
     public boolean allowsPrimaryPromotion() {
         return false;
     }
+
+    @Override
+    public TranslogStats translogStats() {
+        return null; // shadow engine has no translog
+    }
 }