box: loose truncate check in case of foreign key

In #7309 a truncation of a space that was referenced by foreign
key from some other space was prohibited.

It appeared that this solution is too bothering since a user can't
truncate a space even if he truncated referring space before that.

Fix it by allowing space truncate if referring spaces are empty.
Also allow drop of the primary index in the same case with the
same reason: logically the index along with all space data is
not needed for consistency if there's no referring data.

Note that by design space truncate is implemented quite similar
to space drop. Both delete all indexes, from secondary to primary.
Since this patch allows deletion of the primary index (which is
the action that actually deletes all data from the space), this
patch changes the result of space drop too: the space remains
alive with no indexes, while before this patch it remained alive
with no secondary indexes but with present primary. In both cases
the behaviour is quite strange and must be fixed in #4348. To
make tests pass I had to perform drop in box.atomic manually.

Closes #8946

NO_DOC=bugfix

(cherry picked from commit 983a7ec2)