Fix JSON export when TSV contains NaN

Following this user bug report:

This is happening with several series from ECB/GST: the one I’m posting now is just an example.

If you navigate to the series ECB/GST/A.BG.N.B1300.KEX.B0X13.CU.G and ask for the API link, you get the following URL: https://api.next.nomics.world/series?series_ids=ECB%2FGST%2FA.BG.N.B1300.KEX.B0X13.CU.G&format=json which looks correct, but results in an “internal server error”. Weirdly enough, CSV and XSLX exports work ok.