Sampling units files with blank AREA yield no SSUs or TSUs

[rsutphin]

The "Area" feature has turned out not to be that popular with anyone other than the first location that used Navigator. As a result, we'd like to set up sampling units files without an AREA column, or with a blank AREA column.

Unfortunately, when you have a blank or absent AREA column, nothing past the PSU gets extracted from the file. The culprit seems to be the changes made for #7, which skip extracting SSUs/TSUs if AREA is falsy. Fix this.