City/province wiped out in location field

The city or province entered can be wiped out on saving a location field, if either is not present. This is because _location_geo_logic() in location.module looks get the information from the postalcode data if either are not present. But if either are present, they may get removed if the returned data is blank.

The attached patch only fills in the city/province if they were previously blank. (So data entered by the user will not get overwritten.)