Checkmk Werk 14471: Fix possible AttributeError on report rendering

ID: 14471 Title: Fix possible AttributeError on report rendering Component: Reporting & Availability Level: 1 Class: Bug fix Version: 2.2.0i1 If an availability table with annotation using "Reclassify downtime of this period regard as scheduled downtime" was added to a report, the rendering of the report failed with an error like "AttributeError: 'NoneType' object has no attribute 'read'".