Allow empty strings to represent NULL values in export.

Some custom adapters for Sqoop convert null values to empty strings when importing as textfile. This causes failure when the same file is imported back since Sqoop expects the null values to be represented by the string 'null'.

In order to support this use case, Sqoop should allow the interpretation of empty strings as null values provided the datatype is not String. Extending this support for String types will be a problem for databases that do distinguish between null value and empty string.