schema: allow to store smaller field count that specified in format

If a field is not indexed and no more indexed or not nullable
fields after that, than allow to skip it in insertion. Such field
value looks like MP_NIL, but MP_NIL is not explicitly stored.
Named access to this field in lua returns nil.

Example:
format =
{{'field1'},
 {'field2'},
 {'field3', is_nullable = true},
 {'field4', is_nullable = true}}

t = space:insert{1, 2} -- ok.

t.field1 == 1, t.field2 == 2, t.field3 == nil, t.field4 == nil

Closes #2880