Actually, this is a bug/edge case in Onionoo. It reports "country":"" for that relay even though the Onionoo spec says "Omitted if the relay IP address could not be found in the GeoIP database." And while this relay's IP address is contained in the GeoIP file, it's not specified with a country:
Onionoo should simply leave out the country field in this case, just like it doesn't include a country name, region name, or city name. I wrote a patch that I'm testing locally and that I'll apply as soon as it works as expected.
Okay, the Onionoo bug is now fixed in the sense that there is no "country" field for this relay anymore. But Atlas still wants to display this flag as img/cc/.png which obviously doesn't exist. This needs fixing, too. Making this an Atlas ticket again.
Okay, the Onionoo bug is now fixed in the sense that there is no "country" field for this relay anymore.
How it had been fixed? There's also a relay without "country" somehow [1] (and ASN also) while others from the same AS do have this field [2]. It seems to be the same bug.
The first relay you mention above (041DDC5D) does not only lack a "country" field but also all other GeoIP-related fields. That looks like a bug. But I don't (yet) see how that's related to the earlier fix here, because that only affected the "country" field, not the others. I suggest we move the discussion of that new bug to #19154 (moved) and leave this ticket for the Atlas bug of wanting to display img/cc/.png.
I just looked at this ticket again and believe that the only bug that remains is that Atlas shouldn't display the non-existent image file "img/cc/.png" when a relay doesn't have a "country" field. Apparently, this already works in the results table which contains just <td></td> when there's no country flag to display. But the details page tries to put in <dd><img class="inline country" title="" src="img/cc/.png"></dd> which does not exist.
Here's how to reproduce this issue: Run this Onionoo query, scroll down to the first match that doesn't contain a "country" (most do contain one), search for that fingerprint into Atlas, observe how there's no country flag in the results table and broken image on the details page.
Once this Atlas bug is fixed, this ticket can be closed, TTBOMK.