There are two ways to achieve this goal:
In case you just need to apply the default message, all form fields are sent via shortcode:
Click on Submit Button > Other Options
Then enable the Remove Empty Form Input Fields function.
If you want to customize your message content on the Email box, you can apply the remove-line shortcode: [remove_line_if_field_empty]
Click on Submit Button > Email > Message
Copy the field shortcodes that appear inside the fields and paste them or easily use the dynamic tag to render them.
Enter these remove-line shortcodes right behind the single field shortcode.