REST-DSL - Producer fails with NPE or other exceptions if you have not set a hostname

If you forget to configure a host name then the rest producer fails with different errors if you use http4, netty4-http, undertow etc.

We should likely add some validation logic in the rest endpoint / rest producer so it can detect this missing configuration and report this in the same way.

You can reproduce from this example by removing host at
https://github.com/apache/camel/blob/master/examples/camel-example-rest-producer/src/main/java/sample/camel/RestRoute.java#L33