Position.isUndefined() 추가

src/main/java/cokr/xit/base/docs/xls/XLS.java

@@ -205,6 +205,10 @@ public abstract class XLS extends AbstractComponent {
 		public int getCol() {
 			return col;
 		}
+
+		public boolean isUndefined() {
+			return row < 0 || col < 0;
+		}
 	}
 
 	/**이미지 추가 유틸리티