Modify log message to avoid logging null String

ClearcutLoggerApiImpl throws NullPointerException for log message with
null input.

Bug: 30979088
Test: Manually verified message is logged to server.

Change-Id: I0022344a329707b1f8037f9d08e07380b4952772
diff --git a/src/com/android/contacts/common/logging/Logger.java b/src/com/android/contacts/common/logging/Logger.java
index 3ce208e..68cbbbb 100644
--- a/src/com/android/contacts/common/logging/Logger.java
+++ b/src/com/android/contacts/common/logging/Logger.java
@@ -87,11 +87,11 @@
         final Logger logger = getInstance();
         if (logger != null) {
             final QuickContactEvent event = new QuickContactEvent();
-            event.referrer = referrer;
+            event.referrer = referrer == null ? "Unknown" : referrer;
             event.contactType = contactType;
             event.cardType = cardType;
             event.actionType = actionType;
-            event.thirdPartyAction = thirdPartyAction;
+            event.thirdPartyAction = thirdPartyAction == null ? "" : thirdPartyAction;
             logger.logQuickContactEventImpl(event);
         }
     }