If all nodes / edges lack data for an "extra" field, don't include that field in the JSON export

[fedarko]

Case: a GFA graph, when no sequences are given (and they're replaced with a * symbol). All the nodes are assigned a gc_content of None (because to be fair only some nodes could have sequences omitted), but then in the JSON export we have N useless nulls included, and then they show up in the selected node info table...

So we should fix this by adding a check that just identifies cases like this and (for this particular example) omits gc_content from the JSON export, solving this problem.