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.
Added test for DataRow error
Test uses SQLite in-memory DB for test

Also spaces to tabs
Correct code review comments on test
Merge pull request #1172 from roji/datarow_null

Fix DataRow error management