Check if the user has email to avoid errors when sending the message to the user

Problem/Motivation

An error occurred if the user checked Receive email notification for the incoming private messages field and the email field is empty.

Steps to reproduce

- Install the Privatemsg module and enable it.
- Add a new user without email.
- Check the Receive email notification for incoming private messages field.
- Try to send a message to the new user.

Proposed resolution

Check if the user has email to avoid errors when sending the message to the user