On AMP pages when an adserver doesn't respond with an ad, the AMP runtime will collapse the ad as long as it doesn't affect the user scroll position. amp-ad also supports a fallback attribute which is shown when ad isn't available for the slot.

In cases where the ad slot can't be collapsed and the publisher hasn't provided a [fallback] we show blank boxes, which is not good UX. Instead, we should show a default fallback image which has the following components:
a) fallback image with a grey background, similar to Google Contributor

b) Text : "We don't have an ad to show you. Enjoy!"
Text up for debate :)

CC @cramforce @kashyapnitin