Tightened the rules for defining SELECT clauses for a JSON query.

Tightened the rules for defining SELECT clauses for a JSON query.

With two exceptions as noted below, the SELECT list for every class
must be encoded as an array.  Anything else is an error.

The exceptions: null and "*".  For the core class only, these
encodings request a default SELECT list.

For the core class: an empty array requests a default SELECT list.

For a non-core class: an empty array results in an error message,
but not an error.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@13799 dcc99617-32d9-48b4-a31d-7c20da2025e4