Fix for old, broken invariant where unmapped reads are represented by null rather than an empty BAMFileSpan.

git-svn-id: file:///humgen/gsa-scr1/gsa-engineering/svn_contents/trunk@5819 348d0f76-0448-11de-a6fe-93d51630548a

java/src/org/broadinstitute/sting/gatk/datasources/reads/BAMScheduler.java

@@ -94,7 +94,7 @@ public class BAMScheduler implements Iterator<FilePointer> {
             if(currentLocus == GenomeLoc.UNMAPPED) {
                 nextFilePointer = new FilePointer(GenomeLoc.UNMAPPED);
                 for(SAMReaderID id: dataSource.getReaderIDs())
-                    nextFilePointer.addFileSpans(id,null);
+                    nextFilePointer.addFileSpans(id,new GATKBAMFileSpan());
                 currentLocus = null;
                 continue;
             }