CF-4202544 - NULL cannot be made final

Issue: NULL cannot be made final

Steps to Reproduce:
1) Run:

<cfscript>
  final null = null
  writeDump(null)
</cfscript>

Actual Result: coldfusion.runtime.UndefinedVariableException on line 2

Expected Result: [null]

Suggestion: `final null = null` should be allowed, to prevent NULL from being overridden

Verified in build 2018.0.01.308605 (PreRelease).
Filed as CF-4202544.