fix type of R2_md5 (String to String?)

samplesheet.wdl

@@ -4,7 +4,7 @@ struct Readgroup {
     File R1
     String R1_md5
     File? R2
-    String R2_md5
+    String? R2_md5
 }
 
 struct Library {
@@ -70,4 +70,4 @@ task sampleConfigFileToStruct {
         Map[String,Array[Sample]] map = read_json(outputJson)
         Array[Sample] samples = map["samples"]
      }
-}
\ No newline at end of file
+}