I thought about it; looks we won't allow null value there. before this PR, the hdfsmongodb.xml used something like this:

<mongo:converter>
				<bean class="org.springframework.xd.mongodb.TupleWriteConverter">
					<property name="idField" value="${idField:null}"/>
				</bean>
			</mongo:converter>

which sets to idField to "null" string.

What do you think?