Fix NPE with null conversation senders in widget

The ConversationItemView already handled this case

Bug: 6247968
Change-Id: I90849bb10638d7e5c75f607433d5b03f3bfe6be4
diff --git a/src/com/android/mail/widget/WidgetService.java b/src/com/android/mail/widget/WidgetService.java
index baa79b6..81d542a 100644
--- a/src/com/android/mail/widget/WidgetService.java
+++ b/src/com/android/mail/widget/WidgetService.java
@@ -192,7 +192,7 @@
                 // Split the senders and status from the instructions.
                 SpannableStringBuilder senderBuilder = new SpannableStringBuilder();
                 SpannableStringBuilder statusBuilder = new SpannableStringBuilder();
-                senderBuilder.append(conversation.senders);
+                senderBuilder.append(conversation.senders != null ? conversation.senders : "");
                 // TODO: (mindyp) create stylized sender text.
 
                 // Get styled date.