title: Crash when not entering input into textfield
body: <p>I'm trying to figure out this</p>
  
  <blockquote>
    <p>unexpectedly found nil while unwrapping an Optional value</p>
  </blockquote>
  
  <p>Code works fine until I try computeFahreheit Button with an empty UITextField. </p>
  
  <p>Tons of stuff on this subject, I just can't figure it out for this code. </p>
  
  <pre><code>import UIKit
  
  class ViewController: UIViewController {
  @IBOutlet weak var inputCelsius: UITextField!
  @IBOutlet weak var outputFahrenheit: UILabel!
  
  @IBAction func computeFahreheit(_ sender: Any) {
  
      var convertedNumber = Int(inputCelsius.text!)!
      convertedNumber = convertedNumber * 9/5 + 32
      outputFahrenheit.text = "\(convertedNumber)F"
  
      if  (inputCelsius.text == nil) {
          outputFahrenheit.text = "Enter Celsius"
      }else{
          outputFahrenheit.text = "\(convertedNumber)"
      }
  </code></pre>
  
  <p>Any help would be appreciated, thank you.</p>