box: fix autoincrement for json path indexes

The autoincrement code was written when there were no nested field.
Now, it isn't enough to just skip to the autoincrement field - we also
need to descend deeper if key_part->path is set.

Note, the code expects the nested field to be present and set to NULL.
That is, if field path is [1].a.b, the tuple must have all intermediate
fields set: {{a = {b = box.NULL}}} (usage of box.NULL is mandatory to
create a tuple like that in Lua).

Closes #4210