If a column type is not set, null values will be written into persisted batches

The buffer manager will choose the null serializer if a column type is not set. This is not expected to happen, but test code can be sloppy which isn't evident if the weak reference cache is working effectively.

However there is one place in the code that is effected, which is the internally generated partitionId column on any window function expression with a non-empty over clause. This means that on systems with limited memory and or poor performing weak references no results for the window function may be returned.