bug fix if null string is last element in the batch

Author: e-strauss

src/main/java/org/apache/sysds/runtime/util/UnixPipeUtils.java

@@ -514,14 +514,15 @@ private static VarFillResult fillVariableArrayFromBytes(
             if (len < 0) {
                 // null string
                 array.set(offsetOut++, (String) null);
+                bytesConsumed = bb.position();
                 continue;
             }
             if (bb.remaining() < len) {
                 // Not enough bytes for full payload → rollback and stop
                 bb.reset();
                 break;
             }
-
+            
 
             switch (type) {
                 case STRING -> {