message: return null in place of empty string
diff: @@ -365,7 +365,7 @@ function Find-VSWhere {
  Write-Verbose "vswhere.exe location:'$vsWhereLocation'"
  return $vsWhereLocation
  }
  - return ''
  + return $null
  }
  function LocateLatestVSVersionUsingVSWhere {
  @@ -393,7 +393,7 @@ function LocateLatestVSVersionUsingVSWhere {
  return $vsPath
  }
  Write-Verbose "Cannot locate any Visual Studio installation using vswhere.exe".
  - return ''
  + return $null
  }
  function LocateHighestVersionSqlPackageInVS()
  