Check for empty lookup keys in getLookupUri

Bug:19588470
Change-Id: I5379e0b16dbec7692ef86ca8a21b0eae2aa47819
diff --git a/core/java/android/provider/ContactsContract.java b/core/java/android/provider/ContactsContract.java
index 91a19dc..3b4faa3 100644
--- a/core/java/android/provider/ContactsContract.java
+++ b/core/java/android/provider/ContactsContract.java
@@ -1516,8 +1516,14 @@
         /**
          * Build a {@link #CONTENT_LOOKUP_URI} lookup {@link Uri} using the
          * given {@link ContactsContract.Contacts#_ID} and {@link #LOOKUP_KEY}.
+         * <p>
+         * Returns null if unable to construct a valid lookup URI from the
+         * provided parameters.
          */
         public static Uri getLookupUri(long contactId, String lookupKey) {
+            if (TextUtils.isEmpty(lookupKey)) {
+                return null;
+            }
             return ContentUris.withAppendedId(Uri.withAppendedPath(Contacts.CONTENT_LOOKUP_URI,
                     lookupKey), contactId);
         }