PSPossibleIncorrectComparisonWithNull fails to work with typecasted variables

Steps to reproduce

If you validate this script, it complains about PSPossibleIncorrectComparisonWithNull even though it is pretty clear $value must be a string.

Function Get-FooBar() {
    return "foobar"
}

[string]$value = (Get-FooBar)

if ($value -eq $null) {
    Write-Output 'value is $null'
} else {
    Write-Output 'value is not $null'
}

Expected behavior

It works perfectly fine if the value is type-casted like this:

Function Get-FooBar() {
    return "foobar"
}

$value = (Get-FooBar) -as [string]

if ($value -eq $null) {
    Write-Output 'value is $null'
} else {
    Write-Output 'value is not $null'
}

Actual behavior

So we get this error needlessly.

ERROR: test.ps1:9:5: PSPossibleIncorrectComparisonWithNull $null should be on the left side of equality comparisons.

Environment data

> $PSVersionTable
Name                           Value                                                                                               
----                           -----                                                                                               
PSVersion                      6.0.2                                                                                               
PSEdition                      Core                                                                                                
GitCommitId                    v6.0.2                                                                                              
OS                             Linux 4.15.10-1.el7.elrepo.x86_64 #1 SMP Thu Mar 15 10:05:52 EDT 2018                               
Platform                       Unix                                                                                                
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                             
PSRemotingProtocolVersion      2.3                                                                                                 
SerializationVersion           1.1.0.1                                                                                             
WSManStackVersion              3.0     

> (Get-Module -ListAvailable PSScriptAnalyzer).Version | ForEach-Object { $_.ToString() }
1.16.1

Start with the PSPossibleIncorrectComparisonWithNull rule and reproduce the warning using the first PowerShell script, then compare it with the -as [string] example. Trace the rule's handling of explicitly typecast variables and add or update coverage so the warning is not emitted for the demonstrated safe comparison while existing cases remain covered.