Fix registration update processing

Only update registration properties for which a non-null value has beenprovided
My avoiding Validate.notNull stems frmo the fact that we have many, many, many places throughout the code where a check for null resuts in a NullPointerException being thrown, while Validate.notNull throws an IllegalArgumentException in this case. We should settle for one way or the other in order to provide a consistent programming model and API. Right now, the JavaDoc explicitly documents NullPointExceptions being thrown ... we would need to go through the code and fix all occurrences ... Additionally, introducing a dependency on Apache Commons seems to me to be a high price to pay for such a triviality ...