Sets the string representation of a null value. The value specified in this field, if encountered in the source data, will be turned into null .For example: If the source data is as follows:customerId,city,state1, n/a, LA2, Livingston, MIIf we specify n/a for nullValue in the CSV source, then DataMorph/Spark will read the data and replace all the n/a encountered in the data to null as follows:customerId, city, state1, null, LA2, Livingston, MI