Better handling of credentials not configured

Problem / Steps to reproduce

On admin/content/entity_share/pull, after selecting a Remote Website, the overview isn't loaded and the following PHP warning is shown: "Warning: Trying to access array offset on value of type null". This is caused by the getCredentials call in BasicAuth::getJsonApiClient returning null. The Basic Auth credentials are stored locally, so copying over a database from another server to your local development environment will trigger this issue.

Proposed resolution

Check if $credentials contains the keys username and password, and show a warning message if they are not filled in. Perhaps link to the relevant admin/config/services/entity_share/remote/ page.