(GH-2051) Fix: workingDirectory always replaced w/cacheLocation

The previous commit 9e2e49a to address UNC paths broke the ability to
provide a working directory to the Start-ChocolateyProcessAsAdmin
helper function. The test `$workingDirectory.ProviderPath -eq $null`
always evaluates to true because the String type does not have the
member ProviderPath.

The fix is to obtain the location object only when no path is provided
to the helper and to verify its validity before use, possibly falling
back to the cache location, similar to how it was implemented for
Get-ChocolateyUnzip.

src/chocolatey.resources/helpers/functions/Start-ChocolateyProcessAsAdmin.ps1

@@ -118,20 +118,22 @@ param(
   [parameter(Mandatory=$false)][switch] $minimized,
   [parameter(Mandatory=$false)][switch] $noSleep,
   [parameter(Mandatory=$false)] $validExitCodes = @(0),
-  [parameter(Mandatory=$false)][string] $workingDirectory = $(Get-Location -PSProvider 'FileSystem'),
+  [parameter(Mandatory=$false)][string] $workingDirectory = $null,
   [parameter(Mandatory=$false)][string] $sensitiveStatements = '',
   [parameter(ValueFromRemainingArguments = $true)][Object[]] $ignoredArguments
 )
   [string]$statements = $statements -join ' '
 
   Write-FunctionCallLogMessage -Invocation $MyInvocation -Parameters $PSBoundParameters
 
-  if ($workingDirectory -eq $null -or $workingDirectory.ProviderPath -eq $null) {
-    Write-Debug "Unable to use current location for Working Directory. Using Cache Location instead."
-    $workingDirectory = $env:TEMP
+  if ($workingDirectory -eq $null) {
+    $pwd = $(Get-Location -PSProvider 'FileSystem')
+    if ($pwd -eq $null -or $pwd.ProviderPath -eq $null) {
+      Write-Debug "Unable to use current location for Working Directory. Using Cache Location instead."
+      $workingDirectory = $env:TEMP
+    }
+    $workingDirectory = $pwd.ProviderPath
   }
-  $workingDirectory = $workingDirectory.ProviderPath
-
   $alreadyElevated = $false
   if (Test-ProcessAdminRights) {
     $alreadyElevated = $true