restore null check in SequenceUtil.java
restore null check in SequenceUtil.java

annkupi/picard,annkupi/picard,nh13/picard,broadinstitute/picard,nh13/picard,alecw/picard,broadinstitute/picard,broadinstitute/picard,annkupi/picard,broadinstitute/picard,broadinstitute/picard,annkupi/picard,alecw/picard,alecw/picard,alecw/picard,nh13/picard,nh13/picard