Checkmk Werk 13646: Fix host API calls in case a previous discovery failed

ID: 13646 Title: Fix host API calls in case a previous discovery failed Component: REST API Level: 1 Class: Bug fix Version: 2.1.0b6 REST API calls returning host data structures could fail in case a bulk discovery failed for one of the handled hosts before. The API calls failed with a HTTP 500 error and a message like "Server was about to send an invalid response: inventory_failed - Unknown field"