Variable NULL is undefined`

You get an error with

test=null

because, as mentioned in the documentation shared by Charlie, you haven't configured enablenullsupport=true in ColdFusion. The default is enablenullsupport=false.

When enablenullsupport is false, then null is just another name denoting a variable. But the variable does not exist. Which is why you get the error message.

When enablenullsupport is true, then ColdFusion automatically converts the word null on the right-hand-side of the assignment to the null value. I consider this conversion forced and counter-intuitive.

Take the following code. for example.

<cfscript>
try {
	nullTest1=hull;
	nullTest2=javacast("null", 0);
}
catch (any e) {
    //Dump exception 
}
</cfscript>
<p>
nullTest1=null;<br>
nullTest2=javacast("null", 0);	
<p>
<cfoutput>
	<p>	
	isdefined('variables.nullTest1'): #isdefined('variables.nullTest1')#<br>
	isdefined('variables.nullTest2'): #isdefined('variables.nullTest2')#
	</p>	
	isNull(nullTest1): #isNull(nullTest1)#<br>
	isNull(nullTest2): #isNull(nullTest2)#	
</cfoutput>

Run it in turn with this.enableNullSupport = false; in Application.cfc, then with this.enableNullSupport = true;

With this.enableNullSupport = false;, the result is:

Whereas, with this.enableNullSupport = true; the isNull() test results in the exception "Variable NULLTEST1 is undefined.". This is the kind of behaviour I consider counter-intuitive. Which is why I use this.enableNullSupport = false; in my ColdFusion applications.