(GH-724) Set Env Vars to empty should remove vars

Removing environment variables sets empty environment variables. This
is related to the changes for GH-303.

So determine if the value is null or empty, and perform the older
behavior.

src/chocolatey.resources/helpers/functions/Set-EnvironmentVariable.ps1

@@ -15,8 +15,8 @@
 function Set-EnvironmentVariable([string] $Name, [string] $Value, [System.EnvironmentVariableTarget] $Scope) {
 	Write-Debug "Calling Set-EnvironmentVariable with `$Name = '$Name', `$Value = '$Value', `$Scope = '$Scope'"
 
-  if ($Scope -eq [System.EnvironmentVariableTarget]::Process) {
-    return [Environment]::SetEnvironmentVariable($Name, $Value, $Scope)
+  if ($Scope -eq [System.EnvironmentVariableTarget]::Process -or $Value -eq $null -or $Value -eq '') {
+    return [Environment]::SetEnvironmentVariable($Name, $Value, $Scope)
   }
 
   [string]$keyHive = 'HKEY_LOCAL_MACHINE'