Document the purpose of an `if` test and skip coverage

There is not a lot of point in testing this at the moment because the
expected behavior in this case isn't really defined, other than "doesn't
generate an error", and since we have no means of displaying that error,
there is nothing a test can readily check for.

src/sidebar/components/annotation-viewer-content.js

@@ -37,6 +37,12 @@ function AnnotationViewerContent({
       )[0];
 
       if (!topLevelAnnot) {
+        // `annotationId` refers to a reply. We were able to fetch the reply
+        // but not the top-level annotation at the top of the thread.
+        //
+        // We need to decide what what be the most useful behavior in this case
+        // and implement it.
+        /* istanbul ignore next */
         return;
       }