Avoid crash when tagging people in notifications

b/17420805

Change-Id: I93b2b8015761e44195c8a1706b1ade2382b31cbf
diff --git a/src/com/android/mail/utils/NotificationUtils.java b/src/com/android/mail/utils/NotificationUtils.java
index 31c61fa..536a73b 100644
--- a/src/com/android/mail/utils/NotificationUtils.java
+++ b/src/com/android/mail/utils/NotificationUtils.java
@@ -1344,6 +1344,9 @@
         // in the combined notification and add each one as a person.
         if (contactFetcher != null) {
             for (final String sender : sendersList) {
+                if (TextUtils.isEmpty(sender)) {
+                    continue;
+                }
                 final Uri contactLookupUri =
                         contactFetcher.getContactLookupUri(context,
                                 accountName, sender);