groovy expression does not return null

flow.xml

<flow name="myFlow">
  ...
  <jdbc:outbound-endpoint queryKey="insertElement" connector-ref="jdbcConnector" />
  ...
</flow>

groovy-null.xml

<jdbc:connector name="jdbcConnector" dataSource-ref="jdbcDataSource">
  <jdbc:query key="insertElement" value="INSERT INTO MyTable (id, date) VALUES (#[function:uuid], #[groovy:result = (payload.date == null ? null : com.ricston.XmlUtils.toSqlDate(payload.date))])"/>
</jdbc:connector>

groovy-nullpayload.xml

<jdbc:connector name="jdbcConnector" dataSource-ref="jdbcDataSource">
  <jdbc:query key="insertElement" value="INSERT INTO MyTable (id, date) VALUES (#[function:uuid], #[groovy:payload.date == null ? null : com.ricston.XmlUtils.toSqlDate(payload.date)])"/>
</jdbc:connector>

mule-log.log

Exception stack is:
1. Unable to convert between org.mule.transport.NullPayload and JAVA_OBJECT.(SQL Code: 0, SQL State: + 22005) (java.sql.SQLException)
net.sourceforge.jtds.jdbc.Support:533 (null)
2. Failed to route event via endpoint: DefaultOutboundEndpoint{endpointUri=jdbc://insertElement, connector=EEJdbcConnector
...
********************************************************************************
Root Exception stack trace:
java.sql.SQLException: Unable to convert between org.mule.transport.NullPayload and JAVA_OBJECT.
...