Ext.data.Type-optional.js

/*
    Adds an 'optional' property to data type configuration, which allows undefined or null as values.
*/

Ext.iterate(Ext.data.Types, function(key, value, obj) {
    if (value.convert) {
        var fn = value.convert;
        obj[key] = {
            convert: function (v) {
                return (this.optional && (v === undefined || v === null)) ? null : fn(v);
            },
            sortType: value.sortType,
            type: value.type
        };
    }
});

Ext.data.Validations-optional.js

/*
    Adds an 'optional' property to data validation configuration, which allows null as value.
*/

Ext.iterate(Ext.data.validations, function(key, value, obj) {
    if (Ext.isFunction(value)) {
        var fn = value;
        obj[key] = function (config, value) {
            return (value === null && config.optional) || fn(config, value);
        }
    }
});

PoCo.Email.js

Ext.regModel('Email', {
    belongsTo: 'Contact',

    fields: [
        { name: 'value', type: 'string' },
        { name: 'type', type: 'string', optional: true, defaultValue: null },
        { name: 'primary', type: 'boolean'}
    ],

    validations: [
        { type: 'presence', field: 'value' },
        { type: 'email', field: 'value' },
        { type: 'inclusion', field: 'type', list: ['home', 'work', 'other'], optional: true }
    ]
});