Thank you Ernest. I am already using the default result image, which for example on pages with no featured image, correctly show the default image I have set. But for some reason the taxonomy term result doesn’t show the default image. I checked the html output and no div.asp_item_img is output for that item. The taxonomy term has been correctly added to the General Options -> Sources 2. And it shows as the first search result item, including the correct items found count. But no default image shown.
The taxonomy is created by CMB2 plugin. I will investigate if there is a way to set a taxonomy feature image in the code.
In the meantime, is there a filter I can use in functions.php to set an image when none has been found?