Fix DataRow error management

DataRow's RowError is supposed to be String.Empty to indicate
no errors, but is sometimes wrongly set to null, falsely indicating
errors. Fixed to be String.Empty.
Is there a way to test this without a real DB / sqlite, e.g. by using the mocked `MyAdapter` ?
(The test above from which you copied this is currently disabled b.c. of a wrong sqlite version, so we'd likely need to disable this too).

Also, please use tabs instead of spaces.