CF-4202543 - NULL is not final

Issue: NULL is not final

Steps to Reproduce:
1) Run:

<cfscript>
  null = "asdf"
  foo = variables.null
  bar = null
  writeDump(variables)
</cfscript>

Actual Result:

struct
BAR	asdf
FOO	asdf
NULL	asdf

Expected Result:

struct
BAR	[null]
FOO	asdf
NULL	asdf

2) Run:

<cfscript>
  function null() {
    var bar = null
    return bar
  }
  writeDump(null())
  writeDump(variables)
</cfscript>

Actual Result:

function null
Arguments: none ReturnType: Any Roles:   Access: public Output:   DisplayName:   Hint:   Description:

struct
NULL	function null Arguments: none ReturnType: Any Roles:   Access: public Output:   DisplayName:   Hint:   Description:

Expected Result:

[null]

struct
NULL	function null Arguments: none ReturnType: Any Roles:   Access: public Output:   DisplayName:   Hint:   Description:

Suggestions:
1) `bar=null` should always be shorthand for `bar=javaCast("null","")`
2) `var bar=null` should always be shorthand for `var bar=javaCast("null","")`
3) scoping is required, in order to set bar equal to the value of a variable named null (ex: `bar=variables.null` and `var bar=local.null`)

Verified in build 2018.0.01.308605 (PreRelease).
Filed as CF-4202543.