How to send email of filled values only

I have a form that has some optional fields but if a user doesn’t fill them out the label of the input will be sent in the email and they will be empty.
so for example if a user doesn’t enter a city field the email will say
City:

Is there a way to set conditionals where if a user doesn’t enter optional fields the label wont be sent?

http://wordpress.org/plugins/contact-form-7/