Source MV is NULL (from the FIM Portal), How to flow this without the destination attribute being deleted?

Question

Hi all. I had a quick search but couldn't find what I was looking for.
Easy question.
I've setup the following flows:
FIM MA
(FIM)employeeEndDate  -> (MV)employeeEndDate
AD MA
(MV)employeeEndDate  ->(AD)accountExpire (I have a rule extension to convert it to a UTC etc)
If I enter an enddate on a person object in the FIM portal it will flow to the MV and then flow to AD (via the AD CS/MA). 
But (as with all attributes, this is just an example) if I remove the enddate on the person object in the FIM portal it will in turn delete the attribute (AD)accountExpire in the CS of AD. 
This is normal and expected... My question is. How do I flow a NULL and still have the CS attribute retain a value?
I'm not wanting to flow a NULL to the destination but I am taking a NULL in as a source (I am working with rule extensions, I have tried a 'ispresent' but as there's no longer a 'space' to flow to it does nothing, I'm stuck with between flow NULL or do nothing,
depending on if I allow NULLs to flow). 
The reason is the AD attribute accountExpire is never NULL, but either a date or "0" or "9223372036854775807"(Taken from the MSDN page: )
I had thought of using a RE on the import rule of the FIM MA but you can't use RE there.
I hope I've made the question clear, thank you for any help or tips in advanced. 

Answer 1

Try adding a second attribute for your rules extension when exporting from the MV that always has a value (Ctrl + click the other attribute). This will ensure that the rules extension is always fired, something like this:
(MV) accountName
(MV) employeeEndDate --> (AD) accountExpires
From there you should be able to use .IsPresent to check for a value, and flow out the correct value in your rules extension.