DBMS_CLOUD.CREATE_EXTERNAL_TABLE ignoring NULL columns tab delimited file

Summary

External table is ignoring NULL columns on tab delimited file so contents are appearing in wrong columns

Content

I have created an external table over a tab-delimited file with DBMS_CLOUD.CREATE_EXTERNAL_TABLE but I cannot work out how to correctly handle NULL column values. An example will hopefully help. The customer file has the following tab-delimited columns

CUSTOMERID
PERSONID
STOREID
TERRITORYID
ACCOUNTNUMBER
ROWGUID
MODIFIEDDATE

A sample of records from the customer file are;

1 934 1 AW00000001 {3F5AE95E-B87D-4AED-95B4-C3797AFCB74F} 2014-09-12 11:15:07.263000000
2 1028 1 AW00000002 {E552F657-A9AF-4A7D-A645-C429D6E02491} 2014-09-12 11:15:07.263000000
3