"The website encountered an unexpected error" when database indexes are also present

Problem/Motivation

When Search API has configured indexes with the Database backend, a Fatal error is thrown.

Steps to reproduce

1. Configure a site with both a database backend index and a Solr index.
2. Enable the module then visit /solr-health-check
3. A Fatal error is shown (The website encountered an unexpected error. Try again later.).
4. In the PHP error log, the error is:
   

   Uncaught PHP Exception Error: "Call to undefined method Drupal\\search_api_db\\Plugin\\search_api\\backend\\Database::getSolrConnector()" at /var/www/modules/contrib/solr_health_check/src/Controller/HealthCheckController.php line 73

Proposed resolution

It's confusing this error is thrown at all, as the code in question is inside a try ... catch (\Exception $e) block.

Adding a method_exists() check at that line stops the error:

if (method_exists($backend,'getSolrConnector')) {

See attached patch which adds this change.