Specify fileName argument as Nullable.

Bug 20526371.

Change-Id: If53a81f925fbb1b29ad945ba0ec2192d92ae1eff

java/src/com/android/inputmethod/latin/DictionaryStats.java

@@ -36,7 +36,7 @@ public class DictionaryStats {
     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;