code: JcrNodeDefinition getNodeDefinition( NodeDefinitionId definitionId ) {
          if (definitionId == null) return null;
          return nodeTypes().getChildNodeDefinition(definitionId);
      }
docs: Get the node definition given the supplied identifier.
  
  @param definitionId the identifier of the node definition
  @return the node definition, or null if there is no such definition (or if the ID was null)