Add method to check if null handling has been defined

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/csv/trunk@1522878 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -534,6 +534,15 @@ public class CSVFormat implements Serializable {
         return escape != null;
     }
 
+    /**
+     * Returns whether a nullString has been defined.
+     *
+     * @return {@code true} if a nullString is defined
+     */
+    public boolean isNullHandling() {
+        return nullString != null;
+    }
+
     /**
      * Returns whether a quoteChar has been defined.
      *