Only extend a comment if there is a last option

Otherwise an empty line between the question and the options results
in an empty option that appears to the user as a "undefined" option.

gen.pl

@@ -102,7 +102,8 @@ while (<>) {
        "correct" => $1 eq "+" ? JSON::true : $1 eq "-" ? JSON::false : JSON::null
       );
   } elsif (/^.*$/) {		# non-empty line
-    $last_option{"comment"} .= $_;
+    $last_option{"comment"} .= $_
+      if %last_option;
   }
 }