CF-4202534 - variable named NULL spawns in variables scope

Issue: variable named NULL spawns in variables scope

Steps to Reproduce:
1) Enable NULL support
2) Run:

<cfscript>
  myVar = NULL
  writeDump(variables)
</cfscript>

Actual Result:

struct
MYVAR	[null]
NULL	[null]

Expected Result:

struct
MYVAR	[null]

Verified in build 2018.0.01.308605 (PreRelease).
Filed as CF-4202534.

Update: Issue also occurs for URL.myVar=null, FORM.myVar=null, REQUEST.myVar=null, SESSION.myVar=null, APPLICATION.myVar=null and SERVER.myVar=null. It doesn't matter what scope. A variable named `NULL` always spawns into variables scope.