constant for "(null)"

[Ron Grabowski]

Nicko, I think it would helpful to have a constant for when a value
cannot be accessed. Currently the value "(null)" is used but its not
centrally managed. I was using "null" in some code I was writing and
discovered that's inconsistent with what the core uses. It may also be
nice to make this value settable via the config file. If someone is
doing a lot of database work they may want to change "(null)" to
"**NULL**" or "N/A" to avoid confusion with SQL NULL.