Placing a null (i.e. no value) into a cell

Does anyone know of a way to place a null (i.e. no value) in a cell such as:

IFERROR(<<test-value>>,<<null>>)

A great number of the statistical functions, such as CORREL and STDEV exclude a cell in its calculations if it is empty, but obviously not when it is zero, and I want to take a measure on a partially empty list.

Actually, there are a number of times I would like to leave the cell empty after a test, but do not know how to assert it. I do not want to put in an empty string (""), zero, or and error condition. I thought perhaps a function BLANK might do the trick, but there is no such function, just a test for blank (ISBLANK).