AccountIndex: Fix username field

Change-Id: Ie05d36898b426cd52cb5b1e12c245b3651b44623
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/index/account/AccountField.java b/gerrit-server/src/main/java/com/google/gerrit/server/index/account/AccountField.java
index 6da7eef..8627e3a 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/index/account/AccountField.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/index/account/AccountField.java
@@ -16,6 +16,7 @@
 
 import com.google.common.base.Function;
 import com.google.common.base.Predicates;
+import com.google.common.base.Strings;
 import com.google.common.collect.FluentIterable;
 import com.google.common.collect.Iterables;
 import com.google.gerrit.reviewdb.client.AccountExternalId;
@@ -128,10 +129,10 @@
 
   public static final FieldDef<AccountState, String> USERNAME =
       new FieldDef.Single<AccountState, String>(
-            "username", null, false) {
+            "username", FieldType.EXACT, false) {
         @Override
         public String get(AccountState input, FillArgs args) {
-          return input.getUserName().toLowerCase();
+          return Strings.nullToEmpty(input.getUserName()).toLowerCase();
         }
       };