Specify fileName argument as Nullable.

Bug 20526371.

Change-Id: If53a81f925fbb1b29ad945ba0ec2192d92ae1eff
diff --git a/java/src/com/android/inputmethod/latin/DictionaryStats.java b/java/src/com/android/inputmethod/latin/DictionaryStats.java
index 46eaff6..b65d25b 100644
--- a/java/src/com/android/inputmethod/latin/DictionaryStats.java
+++ b/java/src/com/android/inputmethod/latin/DictionaryStats.java
@@ -36,7 +36,7 @@
     public DictionaryStats(
             @Nonnull final Locale locale,
             @Nonnull final String dictType,
-            @Nonnull final String dictFileName,
+            @Nullable final String dictFileName,
             @Nullable final File dictFile,
             final int contentVersion) {
         mLocale = locale;