Avoid CXFInstanceProvider thowing NPE when the endpoint ServiceBean is not available

CXFInstanceProvider throws NPE at line 57 when getServiceBean returns null. That's possible when the user does not provide any endpoint in the jbossws-cxf descriptor. While that's a user mistake, the exception here hides any meaningful message that might be written later on that. So either improve the error here or figure out if it's possible to have the injection simply be skipped here (probably still not a good idea)