When returning a list of bind variables: If a default_ or actual_value

When returning a list of bind variables: If a default_ or actual_value
is undefined for a given variable, leave it out of the JSON object
altogether, rather than create an entry with a JSON null for it.

That way the client can distinguish between an undefined value and
a value defined as null.

M    Open-ILS/src/c-apps/buildSQL.c

git-svn-id: svn://svn.open-ils.org/ILS/trunk@16932 dcc99617-32d9-48b4-a31d-7c20da2025e4