Course with metadata throws NPE

[matthias-ronge]

Describe the bug
A course XML with metadata cannot be imported back. It gives a null pointer exception:

java.lang.NullPointerException: null
    at org.kitodo.production.model.bibliography.course.metadata.CountableMetadata.setStartValue(CountableMetadata.java:306)
    at org.kitodo.production.model.bibliography.course.Course.processRecoveredMetadata(Course.java:385)
    at org.kitodo.production.model.bibliography.course.Course.<init>(Course.java:311)
    at org.kitodo.production.forms.CalendarForm.upload(CalendarForm.java:678)

The line that causes the error was introduced in PR #4924.