CommandBox: extract command text in handleCommandEntered() method

Command text is required twice in handleCommandEntered() method, thus,
the TextInputControl#getText() method is called twice.

This hinders readability.

Let's extract the command text at the start of the method, so that
TextInputControl#getText() is not called repeatedly.

src/main/java/seedu/address/ui/CommandBox.java

@@ -97,12 +97,13 @@ private void replaceText(String text) {
      */
     @FXML
     private void handleCommandEntered() {
-        if (commandTextField.getText().equals("")) {
+        String commandText = commandTextField.getText();
+        if (commandText.equals("")) {
             return;
         }
 
         try {
-            commandExecutor.execute(commandTextField.getText());
+            commandExecutor.execute(commandText);
             initHistory();
             historySnapshot.next();
             commandTextField.setText("");