sql_io.py: func_wrapper_exception_handler(): Use new sql_gen.merge_not_null() to try to ensure that NULL values are not folded (which would cause the concatenated values not to match up with the concatenated column names). Note that this adds a dependency on the db object, which callers must now provide.

sql_gen.py: Added merge_not_null()

sql_gen.py: Added try_mk_not_null()

sql_gen.py: Renamed ArrayJoin to ArrayMerge to avoid confusion with Join (a SQL construct)

sql_io.py: put_table(): is_function: Creating error handling wrapper function: Set srcs on row_var so that the column type and nullability info of row_var's columns can be retrieved for use with sql_gen.ensure_not_null()

sql_gen.py: RowExcIgnore.to_str(): Compare self.row_var to global const row_var using == to allow caller to provide a copy of row_var with the underlying table set appropriately

sql_gen.py: underlying_table(): Support derived tables and row vars by obtaining the underlying table from the srcs

sql_io.py: put_table(): Setting pkeys of missing rows: Fixed bug where also needed to do this when is_function if an empty pkeys table was created (due to an error that could not be localized to a row)

sql_io.py: put_table(): After main loop: If is_literals, return immediately to avoid needing to test for is_literals in all the code that follows (which only applies to the normal case)

sql_gen.py: RowExcIgnore: If a custom row_var is used, require it to already be defined. This also allows sql_io.ExcToErrorsTable to place the column var definition in the outer DECLARE, eliminating the extra DECLARE block.