Placeholder for empty display_name

In the file `wp-job-manager-alerts/includes/emails/class-job-alert-email.php` I'd suggest line 352 is changed from:

`'{display_name}' => $user->display_name ?? $user->user_email,`
to
`'{display_name}' => !empty($user->display_name) ? $user->display_name : 'there',`

...this will stop an alert going out to a user without a display name set (for whatever reason) and rendering a space where the display name would be:

**Before:**
"Hi ,"

**After:**
"Hi there,"

Open wp-job-manager-alerts/includes/emails/class-job-alert-email.php and inspect line 352 and the surrounding email data mapping. Verify the job alert greeting when a user has no display_name; the work is done when the greeting uses “there” instead of rendering a blank name.

Written by the indexing model from the issue text.