Emit null instead of empty object for validateAshHooks

[foolip]

This was left to minimize report.json changes in earlier
refactoring. Fixing it now should only affect the report.json
structure but not report.html.

[foolip]

I've trimmed this PR to only the remaining TODO.

[dontcallmedom]

The diff shows

-      {
-        "repo": "w3c/dnt"
-      },
-      {
-        "repo": "w3c/qt3tests"
-      },
-      {
-        "repo": "w3c/web-annotation"
-      }
+      "w3c/dnt",
+      "w3c/qt3tests",
+      "w3c/web-annotation"
     ],

which I don't understand and wouldn't have expected. Any insight?

[foolip]

Oh, I didn't anticipate this, but it's because of this logic:

validate-repos/validate.js

Lines 55 to 61 in 39443ae

	// Push just the repo name string if there are no details, and otherwise
	// add it as a `repo` property to the details object.
	if (details === null) {
	allerrors[type].push(fullName(repo));
	} else {
	allerrors[type].push({repo: fullName(repo), ...details});
	}

@dontcallmedom maybe this should be changed to always use the object form? If such a change is landed first, this change should then be a no-op.

[dontcallmedom]

changing to always use object form makes sense to me, but will probably need changes in downstream usage of the JSON results (which points toward the need of setting up a JSON schema for the results I think)

[foolip]

As it happens I was just working on using a JSON schema to validate w3c.json, if that works out it should be straightforward (ish) to use it for report.json as well.

Let's leave this PR hanging, it's not urgent in any way.