Only send update notification email when there is an email address

Problem/Motivation

After deleting the email listed under (/admin/reports/updates/settings), each time a cron run is started, the following error is created:
Error sending email (from info@website.de to with reply-to not set)

Steps to reproduce

go to: /admin/reports/updates/settings

1. Empty the list of email addresses
2. After the system finds a new update, based on your configuration of "Check for updates" and "Email notification threshold"
3. Run cron

The error disappears after adding back an Email into the notify list. But will reappear if it the email is delated again.

Doing a single export on update.settings.yml results in: 

notification: 
  emails: 
    - ''

On a different website where this error doesn't appear we get: 

notification:
  emails: { }

Here the notification email was never set.

Proposed resolution

Send update email only if there is an email address.

Remaining tasks

None

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A