I have a function, 

    f[dist_, samp_]:=somework[dist, samp]

that I want to return `Null`, or zero if passed a null distribution. I can't find a way to test for a difference between `Null` and Data distribution, none of  `===`, `=!=`, `SameQ` and `==` can identify them.

How can I tell if I'm being passed a `Null` value to handle it correctly?