NameItem::mainPropertyName() returns 'value' which is not a property of the name field

Problem/Motivation

The NameItem field item class does not override mainPropertyName() and thus returns 'value'. 'value' does not respond to a property on name field items so any code that makes use of getMainPropertyName() breaks or gives incorrect results.

One example of this is in CTools TypedDataRelationshipDeriver:

if ($main_property = $property_definition->getFieldStorageDefinition()->getMainPropertyName()) {
        if ($property_definition->getFieldStorageDefinition()->getPropertyDefinition($main_property)) {

Solution
Correctly override mainPropertyName to return NULL