Edge case in the Field API click sort code

Just ran into an interesting bug.

If you add a field, and then hit "Cancel", the click sort column option doesn't get saved. The default might be an empty string.
And the click sort function tries to use that option even if it's empty, and fails (taking the query down with it).

Two changes:
1) Abort the click sort if the column is empty
2) Better detecting of column defaults (if there is only one column, use it as the default)