box: always set index.parts.exclude_null to true or false

Currently the `exclude_null` field of `index_object.parts` is inconsistent
between local box and net.box connection. It returns the following values:
- locally: true / null
- net.box: true / false

This mismatch makes it difficult to compare schemas on the storage and via
the net.box connection on the router (see tarantool/crud#361).

Closes #8649

NO_DOC=minor