Error "Empty speech response"

This happened to me. If this happens for an Intent you just added in the Dialogflow console and you are using Webhook fulfillment for the action, check the intent's fulfillment settings and ensure that the Webhook fulfillment slider is on. Evidently new intents don't automatically get webhook fulfillment: you have to opt each one in piecemeal (or at least, that was my experience).

I experienced this situation too. My problem was that I used a SimpleResponse in my fulfillment index.js without referencing to it. So the solution for me was to add SimpleResponse like this in index.js:

const {dialogflow, SimpleResponse} = require('actions-on-google');

So, always check if you aren't using any dependencies without including it in your js-file. Probably not the most common cause of the problem, but it can be.