title: IsNullOrEmpty' does not exist in the current context
body: <p>I have this code:</p>
  
  <pre><code>public static FormattedString AddParagraph(this FormattedString formattedString, string text)
      {
          if (IsNullOrEmpty(text))
              return formattedString;
          else
          {
              formattedString.Spans.Add(new Span { Text = text + Environment.NewLine + Environment.NewLine, ForegroundColor = Color.FromHex("555555") });
              return formattedString;
          }
      }
  </code></pre>
  
  <p>But it's telling me: The name 'IsNullOrEmpty' does not exist in the current context </p>
  
  <p>Can anyone give me advice on how I can check if this string is "" or it has something other than ""?</p>