FOP-3120: IllegalArgumentException for list in a table

fop-core/src/main/java/org/apache/fop/layoutmgr/AbstractLayoutManager.java

      * @return True if it is the first Position
      */
     public boolean isFirst(Position pos) {
-        //log.trace("isFirst() smallestPosNumberChecked=" + smallestPosNumberChecked + " " + pos);
+        if (pos == null) {
+            return false;
+        }
         verifyNonNullPosition(pos);
         if (pos.getIndex() == this.smallestPosNumberChecked) {
             return true;
      * @return True if it is the last Position
      */
     public boolean isLast(Position pos) {
+        if (pos == null) {
+            return false;
+        }
         verifyNonNullPosition(pos);
         return (pos.getIndex() == this.lastGeneratedPosition
                 && isFinished());