FieldDef: Mark Getter#get as nullable

Change-Id: I6bb474883e2657aaa8effcc5600681426a484dcf
diff --git a/java/com/google/gerrit/index/FieldDef.java b/java/com/google/gerrit/index/FieldDef.java
index beb9c07..f565e52 100644
--- a/java/com/google/gerrit/index/FieldDef.java
+++ b/java/com/google/gerrit/index/FieldDef.java
@@ -18,6 +18,7 @@
 import static java.util.Objects.requireNonNull;
 
 import com.google.common.base.CharMatcher;
+import com.google.gerrit.common.Nullable;
 import com.google.gwtorm.server.OrmException;
 import java.io.IOException;
 import java.sql.Timestamp;
@@ -60,6 +61,7 @@
 
   @FunctionalInterface
   public interface Getter<I, T> {
+    @Nullable
     T get(I input) throws OrmException, IOException;
   }
 
@@ -133,6 +135,7 @@
    * @return the field value(s) to index.
    * @throws OrmException
    */
+  @Nullable
   public T get(I input) throws OrmException {
     try {
       return getter.get(input);