Clean up partcollation handling for OID 0.

Clean up partcollation handling for OID 0.

Consistent with what we do for indexes, we shouldn't try to record
dependencies on collation OID 0 or the default collation OID (which
is pinned).  Also, the fact that indcollation and partcollation can
contain zero OIDs when the data type is not collatable should be
documented.

Amit Langote, per a complaint from me.

Discussion: http://postgr.es/m/CA+Tgmoba5mtPgM3NKfG06vv8na5gGbVOj0h4zvivXQwLw8wXXQ@mail.gmail.com