The attribute not_null_tables could be calculated incorrectly in the
function SELECT_LEX::update_used_tables for queries over views 
with row items in the WHERE clause. It happened because no 
implementation of the virtual callback function eval_not_null_tables
was provided for the class Item_row.
Also slightly optimized the code calculating the value of the maybe_null
flag for tables in the function SELECT_LEX::update_used_tables.