Distinguish between "null" and actual null in getSamplePlotData?

[fedarko]

If the user passes in an empty value in the sample metadata, it'll get stored throughout Qurro as a plain null value. However, it's also allowed for the user to have e.g. a string of "null" that is passed in -- and this is stored as a string, not an actual null.

The complication comes when the user goes to export the data from the sample plot in the viz interface. I haven't been able to test this yet, but I suspect that both forms of null mentioned above will yield identical results in the exported data.

This isn't really a bug, but I can see how it would be confusing (esp since invalid log ratios are also represented as null in the output data).

Maybe a reasonable solution would be quoting String nulls in the exported data? And/or just making the docs better, I guess.