Update picard.wdl

Co-authored-by: DavyCats <davycats.dc@gmail.com>

picard.wdl

@@ -132,7 +132,7 @@ task CollectMultipleMetrics {
         File? qualityDistributionPdf = basename + ".quality_distribution.pdf"
         File? qualityYield = basename + ".quality_yield_metrics"
         # Using a glob is easier. But will lead to very ugly output directories.
-        Array[File]? allStats = select_all([
+        Array[File] allStats = select_all([
             alignmentSummary,
             baitBiasDetail,
             baitBiasSummary,