"In the following code sample, the containing img and is appropriately labeled by the caption paragraph. In this example the img element can be marked as presentation because the role and the text alternatives are provided by the containing element."
https://w3c.github.io/aria/#example-13
"In the following code sample, the containing img and is appropriately labeled by the caption paragraph. In this example the img element can be marked as presentation because the role and the text alternatives are provided by the containing element."
<img src="example.png" role="presentation" alt="">
So empty alt tags are not discouraged.