Only throws an error when there are no samples but there are reads

-- Handles the case when you are running a ROD traversal and yet the LIBS is still used to return null everywhere.

public/java/src/org/broadinstitute/sting/gatk/iterators/LocusIteratorByState.java

@@ -280,7 +280,9 @@ public class LocusIteratorByState extends LocusIterator {
         this.samples = new ArrayList<String>(samples);
         this.readStates = new ReadStateManager(samIterator,readInformation.getDownsamplingMethod());
 
-        if ( this.samples.isEmpty() )
+        // currently the GATK expects this LocusIteratorByState to accept empty sample lists, when
+        // there's no read data.  So we need to throw this error only when samIterator.hasNext() is true
+        if ( this.samples.isEmpty() && samIterator.hasNext() )
             throw new IllegalArgumentException("samples list must not be empty");
     }