title: Swift3 UITextField in UIAlert validation after button pressed
body: <p>I have implemented alert window with two text fields. And I want to perform some validation: if one of text fields is blank (user did not input any value), application should not allow user to press the "Done" button.
  I don't want to show any alerts, just want to not allow user to save blank data.</p>
  
  <p>I have created the function listed below. Added two guards with return. But in this case if user did not input anything, alert just closes and nothing is saved. I don't want alert window to be closed.</p>
  
  <p>How can it be done if it can be? Have not found the answer. I have checked <a href="https://stackoverflow.com/questions/44964838/how-to-dismiss-alert-view-after-validation">this</a> question, but looks like it's not applicable for me. Appreciate your help!</p>
  
  <pre><code>private func addTable() {
      let alert = UIAlertController(title: NSLocalizedString("inputTableParams", comment: ""), message: nil, preferredStyle: .alert)
      alert.addTextField(configurationHandler: configureTableNameTextField)
      alert.addTextField(configurationHandler: configureTableCapacityTextField)
      alert.textFields?[0].autocapitalizationType = .sentences
      alert.textFields?[1].autocapitalizationType = .sentences
      alert.addAction(UIAlertAction(title: NSLocalizedString("alertCancel", comment: ""), style: .cancel, handler: nil))
      alert.addAction(UIAlertAction(title: NSLocalizedString("alertDone", comment: ""), style: .default, handler: { (UIAlertAction) in
          guard self.tableNameTextField.text != "" else {return}
          guard self.tableCapacityTextField.text != "" else {return}
          let newTable = Table(tableName: self.tableNameTextField.text!, tableCapacity: Int(self.tableCapacityTextField.text!)!)
          let result = try? TablesTable.getOrCreateTable(table: newTable)
          if result != nil {
              self.updateTableView()
          }
      }))
      self.present(alert, animated: true, completion: {})
  }
  </code></pre>