fix : null values in arrays were wrongly persisted as empty strings

[loichenninger]

Bug: When null values were present in arrays in the raw data, the API persisted them as arrays of empty strings in the document with a COLLECTED state. Sometimes, if the array was supposed to contain numeric values, the API also created an additional document in the FORMATTED state, which again contained empty strings.

This caused issues in client applications. They need to compare the initial state (COLLECTED) with the EDITED state (if it exists). But in our case, the initial state was altered: it looked as if we had received an empty string "", whereas in reality we had received a null value.

Solution: We no longer persist table values in the COLLECTED document when they are null. The client applications will interpret that as a null value in the collected data.