Bug fix in previous commit

We need to gather $value from $prevEntry before we can determine if it
is empty or not.

if(!$value) at the top of that block refers to whether a value came back
for this element, from the form submission that led to this page, or
not.

Also amended the logic so required fields are treated this way also.  A
required field, by definition, should not be empty in the database, so
if something has happened to make it empty, we'll ignore that blank
value and use the default instead.
Moving new option to Advanced tab

Since required fields are now treated the same way, it makes sense for
this additional option to be declared "Advanced" since it would be used
in fewer circumstances.