Invoking HstComponentParameters#setValue with value null results in broken hst component configuration

When invoking org.hippoecm.hst.pagecomposer.jaxrs.util.HstComponentParameters#setValue with value 'null', the parameter should be removed instead. Now, the result is that the prefixes and parameter names have one item more than parameter values resulting in a broken hst component config. For simplicity, also add org.hippoecm.hst.pagecomposer.jaxrs.util.HstComponentParameters#remove