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