result_set_builder's API is:
new_row
add
add
add
new_row
add
add
add
new_row
add
add
add
build
Since there is no end_row, it relies on an internal flag to see (in new_row
and in build) whether we need to end a previous row. The problem is that
we check if the row is empty(), which is true both for the first row, and
for an empty row (if add() is never called, e.g. "SELECT COUNT(*) FROM tab".
Fix by using optional<> to mark whether the row exists (new_row has been
called). This is ugly, but matches origin. We should improve that by
adding an explicit end_row().
To prevent name clashes, we don't call the virtual function implementing
this to_string(), but rather assignment_testable_source_context(), as its
use will be error reporting.
