Fix variable detection on linux

Linux can have empty values for environment variables. Windows evidently doesn't tend to have those.

azure-pipelines/variables/_pipelines.ps1

@@ -14,7 +14,7 @@ param (
 (& "$PSScriptRoot\_all.ps1").GetEnumerator() |% {
     # Always use ALL CAPS for env var names since Azure Pipelines converts variable names to all caps and on non-Windows OS, env vars are case sensitive.
     $keyCaps = $_.Key.ToUpper()
-    if (Test-Path -Path "env:$keyCaps") {
+    if ((Test-Path "env:$keyCaps") -and (Get-Content "env:$keyCaps")) {
         Write-Host "Skipping setting $keyCaps because variable is already set to '$(Get-Content env:$keyCaps)'." -ForegroundColor Cyan
     } else {
         Write-Host "$keyCaps=$($_.Value)" -ForegroundColor Yellow