sql: workaround. Allow NILs comparison for scalars.

     * Allow NIL to belong to `scalar` class.
     * Extend comparator to always return false when comparing with `NIL`s.
     * Should be removed as fas as #1557 is implemented.

sql: Fix expected result in engine/indices_any_type.test.lua.

	test/engine
	* indices_any_type.result: Update expected result as comparison
	against nulls changed.