dev/core#3028 - For invalid greetings, return '' instead of failing

[totten]

Overview

Fix crash when re-generating greetings for invalid data.

As mentioned in the ticket, I was able to reproduce by hacking at the SQL:

update civicrm_contact set postal_greeting_id = 999 where id = 200;

I think a plausible, organic scenario for this would be:

1. Add a new postal_greeting (Ex: INSERT INTO civicrm_option_value ... ("My Greeting",123,...))
2. Update a Contact to use the new greeting. (Ex: Set postal_greeting_id=123)
3. Delete the postal_greeting. (Ex: The contact now has an invalid postal_greeting_id==123.)
4. Perform some action that requires recalculating the greeting. (Ex: Merge the contact with another contact.)

See: https://lab.civicrm.org/dev/core/-/issues/3028

Before

Recalculating the greeting leads to a type error:

TypeError: Return value of CRM_Contact_BAO_Contact::getTemplateForGreeting() 
must be of the type string, null returned in CRM_Contact_BAO_Contact::getTemplateForGreeting()
(line 3509 of /var/www/vhosts/xyz/webroot/sites/all/modules/civicrm/CRM/Contact/BAO/Contact.php).

After

No crash. The greeting value is blank (which is a common form omission, if you look at other paths in the context).

[civibot]

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[totten]

This is actually @eileenmcnaughton's patch which I had tested last night. Adding "merge on pass" based on last night's testing.