This line should resolve issue #428 (readNWISstat failing on empty returns)

This simply states not to join the data if there is no data to join.

It also resolves a similar error I had:
readNWISstat(siteNumbers = '02108000',
              parameterCd = "00600",
              startDate = "1800-01",
              endDate = format(Sys.time(), "%Y-%m"),
              statReportType="monthly",
              statType="mean")