Fix styles in setting proxy

Change-Id: Iac4243e767a7cfb29b9bb0b4603ca6509356ce8c
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Heikki Halmet <heikki.halmet@qt.io>

coin/provisioning/common/windows/set-proxy.ps1

@@ -35,7 +35,7 @@ $n = Get-Content "$PSScriptRoot\..\shared\http_proxy.txt"
 $n = $n.Split('=')
 New-Variable -Name $n[0] -Value $n[1]
 
-if (([string]::IsNullOrEmpty($proxy)) -or ($proxy -eq '""')) {
+if ([string]::IsNullOrEmpty($proxy)) {
     Write-Host "No proxy is defined."
 } else {
     Write-Host "Checking proxy @ $proxy"