Simple Code Question - Checking strings for NULL

I was never taught this one, but it seems simple and vital...

I have a function that CAN be passed the name of an object, but doesn't require it:

function makeTestObject(%mesh,%tex,%end,%child)
//%mesh = which mesh do we use? integer value 1-10
//%tex = which texture to give our mesh? integer value 1-5
//%end = Z position where we want our object to stop
//%child = which object do we activate once we are done with this one? string value = name of object

So then later, I have code that can move that test object to a specific Z position. When the object reaches that position, I want to start moving the child, but only if one exists. I know I can do simple things like:

activate(TestObject.child);

Assuming that 'activate' is a valid function and my "makeTestObject" used %child as such:

TestObject.child = %child;

Which it does...

But, what if %child was left blank? Is this ok? I am thinking the worst that will happen is that I will get a console error: Cannot find object ''. Attempting to call function 'activate'. (Something like that).

What would be nicer though would be something like:

if(TestObject.child != NULL) activate(TestObject.child);


Only thing is, that line doesn't work. So, how do I check for a NULL value in a string variable?