Check for existence of a public contact id before deleting related notifications in Model\Contact

- Address part of https://github.com/friendica/friendica/issues/11993#issuecomment-1338134893

src/Model/Contact.php

@@ -3209,9 +3209,10 @@ class Contact
 		self::clearFollowerFollowingEndpointCache($contact['uid']);
 
 		$cdata = self::getPublicAndUserContactID($contact['id'], $contact['uid']);
-
+		if (!empty($cdata['public'])) {
 			DI::notification()->deleteForUserByVerb($contact['uid'], Activity::FOLLOW, ['actor-id' => $cdata['public']]);
 		}
+	}
 
 	/**
 	 * Update the local relationship when a local user unfollow a contact.