Fixes https://github.com/spring-projects/spring-integration/issues/3688
The `WebServiceTemplate` is populated with some defaults from its ctor.
We should rely on the target default template values as much as possible
and don't override them to `null` if end-user doesn't ask about that explicitly
* Fix `BaseWsOutboundGatewaySpec` extensions to populate values to the target gateway
only if they are not null - therefore provided by end-user
* If end-user wants them explicitly `null`, it is better to do that via
externally configured template.
See overloaded variants for DSL: `Ws.marshallingOutboundGateway(WebServiceTemplate)`
and `Ws.simpleOutboundGateway(WebServiceTemplate)`
**Cherry-pick to `5.4.x` & `5.3.x`**
db14faddd1