Mentorship decline email says the mentor has provided a reason even if they don't

When mentors decline a request they allowed to not specify the reason.

**The issue**

Currently, even if they didn't leave a reason, the email says
> They provided the reason below, which we hope will help you find your next mentor:

**The solution**

Don't include the following part if the `reason` is empty

https://github.com/Coding-Coach/find-a-mentor-api/blob/4a68ae93fe7e723cab3ca1b342556a868224bc7e/content/email_templates/mentorship-declined.html#L29-L35