global class EmailHandler implements QuickAction.QuickActionDefaultsHandler {
    // Empty constructor
    global EmailHandler() {
    }
    // The main interface method
    global void onInitDefaults(QuickAction.QuickActionDefaults[] defaults) {
        System.debug(defaults);
        QuickAction.SendEmailQuickActionDefaults sendEmailDefaults = null;
        // Check if the quick action is the standard Case Feed send email action
        // and set sendEmailDefaults
        for (Integer j = 0; j < defaults.size(); j++) {
            sendEmailDefaults = (QuickAction.SendEmailQuickActionDefaults)defaults.get(j);
            System.debug('Email Defaults For Loop ' + j + ': ' + sendEmailDefaults);
            break;
        }

        if(sendEmailDefaults != null) {
            //select the relevant case
            Case c = [SELECT CaseNumber, Source__c, CC_Team__c, Subject, OwnerId
                      FROM Case WHERE Id=:sendEmailDefaults.getContextId()];
            System.debug('Case ID: ' + c.Id);
            //select the user that owns the above case
            User u = [SELECT FirstName, LastName FROM User WHERE Id=:c.OwnerId];

            EmailMessage emailMessage = (EmailMessage)sendEmailDefaults.getTargetSObject();

            //this will clear certain fields if using the centre console
            if (sendEmailDefaults.getActionName() == 'Case.CC_Send_Email') {
                emailMessage.ToAddress = '';
                emailMessage.CcAddress = '';
            }
            System.debug('To Address after clean: ' + emailMessage);
        }
    }
}

@isTest
private class EmailHandlerTest {
    static Case myCase {get;set;}
    static EmailMessage myMsg {get;set;}

    static testmethod void EmailPublisherLoader_NoReplyToId() {
        Exception failureDuringExecution = null;
        init();

        //create QuickActionDefaults
        List<Map<String, Object>> defaultSettingAsObject = new List<Map<String, Object>>
        {
            new Map<String, Object>
            {
                'targetSObject' => new EmailMessage(),
                    'contextId' => myCase.Id,
                    'ToAddress' => myMsg.ToAddress,
                    'CCAddress' => myMsg.CcAddress,
                    'actionType' => 'Email',
                    'actionName' => 'Case.CC_Send_Email',
                    'fromAddressList' => new List<String> { '[email protected]' }
            }
        };

            List<QuickAction.SendEmailQuickActionDefaults> defaultsSettings =
            (List<QuickAction.SendEmailQuickActionDefaults>)JSON.deserialize(JSON.serialize(defaultSettingAsObject), List<QuickAction.SendEmailQuickActionDefaults>.class);

        Test.startTest();
        (new EmailPublisherLoader()).onInitDefaults(defaultsSettings);
        System.debug('defaultsSettings after: ' + defaultsSettings);
        Test.stopTest();
        //System.assertEquals(null, defaultsSettings.QuickActionDefaults.targetSObject.ToAddress);
    }

    static void init(){
        myCase =
            new Case(
                Status='Unassigned',
                Priority = 'Medium',
                Origin = 'Email',
                RecordTypeId = '012800000007qIgAAI'
            );
        insert myCase;

        myMsg =
            new EmailMessage(
                ToAddress = '[email protected]',
                CCAddress = '[email protected]',
                ParentId = myCase.Id
            );
        insert myMsg;
    }
}

10:27:36:053 USER_DEBUG [35]|DEBUG|defaultsSettings after: (SendEmailQuickActionDefaults:[QuickActionDefaults.actionName=Case.CC_Send_Email, QuickActionDefaults.actionType=Email, QuickActionDefaults.contextId=50023000001yVHlAAM, QuickActionDefaults.targetSObject=EmailMessage:{ToAddress=}, emailTemplateId=null, fromAddressList=(), ignoreTemplateSubject=null, insertTemplateBody=null, replyToId=null])