Kie server on WebSphere 9 doesn't correctly process Content-Type header

If user doesn't fill Content-Type header for REST requests to Kie server then error is returned:
"org.apache.cxf.interceptor.Fault: Name is null".
This error is thrown even for GET requests where this header isn't required.

Problem is in [1], WebSphere provides list of Content-Type headers as SingletonList with null value. This list has constant size 1, so isEmpty() method returns false and null is used as content type.

[1] https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-remote/kie-server-rest/kie-server-rest-common/src/main/java/org/kie/server/remote/rest/common/util/RestUtils.java#L120