sql: Allow NULL values in columns.

     * If `NULL` is allowed for column - demote column affinity
     to `scalar` in Tarantool.
     * Set `unique` Tarantool's property to false if any of
     index columns may contain `NULL`.
     * If column may not contain NULL then use OP_Rewind/OP_last
     to fetch first(last) indexed element during searchg for MAX/MIN.
     * Add tests for MIN() and MAX(), when value is indexed.

sql: Fix field extraction via field_map

Fix the crash when encountered a tuple with outdated field_map.