map a null/missing variable?

[AJ Weber]

OK,

So I'm still working on my caching "issue", but this is a more general 
question, so for the sake of indexing and helping others in the future 
with (hopefully) a response...

How do I use a map to map the lack of a variable/header/cookie (NULL?) 
to a value?  I can't use "default", because when there IS a value it 
will be a random set of characters.

So that said, if a regex like ".+" would work for "any value" and I use 
'default' for the NULL, I would try that. :)

Thanks in advance,
AJ