feature(wakeup): exit if no matching computers found

wakeup.ps1

@@ -75,6 +75,13 @@ $targetComputers = [System.Collections.Generic.List[PSObject]]::new()
 $db.WOLDatabase.Computers.Computer | Where-Object { $_.name -match "$targetComputer" } | ForEach-Object { $targetComputers.Add($_) }
 $removeFromTargetComputers = [System.Collections.Generic.List[String]]::new()
 
+# exit if nothing to do (i.e. empty targetComputers list)
+if ($targetComputers.Count -eq 0)
+{
+    Write-Host -ForegroundColor Yellow "No computers found matching '$targetComputer'. Nothing to do.`r`n"
+    exit 0
+}
+
 # send WOL magic packets
 $targetComputers | ForEach-Object {
     $wolError = @()