conversation3.js

class Conversation {

    //...

    //GREETING_EXISTING_USER starts with "Hey%s! ...". So you can put there a name with preceding space, 
    //or simply an empty String
    _greetExistingUser() {
        const loggedWaterPromise = this.waterLog.getLoggedWaterForUser(this._getCurrentUserId());
        const userNamePromise = this.userManager.loadAssistantUser(this._getCurrentUserId());

        return Promise.all([loggedWaterPromise, userNamePromise])
            .then(values => {
                const loggedWater = values[0];
                const userGivenName = values[1].givenName;
                let formattedName = '';
                if (userGivenName) {
                    formattedName = ' ' + userGivenName;
                }
                this.dialogflowApp.ask(
                    util.format(Str.GREETING_EXISTING_USER, formattedName, loggedWater),
                    Str.GREETING_EXISTING_USER_NO_INPUT_PROMPT
                );
            });
    }
}