fix chunking for different result types

We should be able to fix the commit chunking for different result_types by setting result_number, result_string and result_boolean to None before assigning the observation.value.

If we don't do this, then sqlalchemy is throwing a compile error:

sqlalchemy.exc.CompileError: INSERT value for column observation.result_number is explicitly rendered as a boundparameter in the VALUES clause; a Python-side value or SQL expression is required

Which again should be fixed by providing None as the python-side value.