Formulate with condition if measure is blank

I'm trying to subtract one measure from another only if the second measure has a value.  If not, the formula should return empty.  The below is not working.  MDX ISEMPTY is not available in Formulate nor is ISBLANK.  Not sure what to use here.  Thanks.

Current formula:

Difference from Default =
IF(
    IsNAN([measures].[Events_DefaultAmount]),
    NULL,
    [measures].+[fc2a0693-8321-4537-95c4-68e0848127f4]-[measures].[Events_DefaultAmount]
)

 

Desired result:

Product	[measures].+[fc2a0693-8321-4537-95c4-68e0848127f4]	][measures].[Events_DefaultAmount]	Difference from Default
Product1	25	15	10
Product2	21
Product3	12	20	-8
Product4	4
Product5	55	45	10