next() Called On Possible Empty iterator()

Consider this function:

 protected void reduce ( VarIntWritable index, Iterable<VarLongWritable> ids, Context
   ctx) throws IOException, InterruptedException {
     ctx.write(index, ids.iterator().next());
 }

The line `ctx.write(index, ids.iterator().next());` does not check for empty iterator when calling the next() function, which could result in an exception.

Function can be found on the GitHub repository here.

Line 395, in file /src/main/java/org/apache/mahout/cf/taste/hadoop/als/ParallelALSFactorizationJob.java