Fixed NPE.

Author: rxin

unsafe/src/main/java/org/apache/spark/unsafe/types/UTF8String.java

@@ -402,6 +402,10 @@ public UTF8String lpad(int len, UTF8String pad) {
    * For example, concat("a", null, "c") would yield "ac".
    */
   public static UTF8String concat(UTF8String... inputs) {
+    if (inputs == null) {
+      return fromBytes(new byte[0]);
+    }
+
     // Compute the total length of the result.
     int totalLength = 0;
     for (int i = 0; i < inputs.length; i++) {