Fix the empty data-table edge-case historical API handling

[AlexLipp]

Sometimes the ThamesWater API returns empty data-tables. This may sometimes correct, but in general it is likely incorrect. This should therefore prompt a retry.

[AlexLipp]

PR #16 has ameliorated this issue significantly but one edgecase remains. If there have been exactly an integer multiple of the API limit number of events (e.g., 0, 1000, 2000) an empty data table will be returned resulting in a false exception. It is not entirely clear how to fix this edgecase however as its very hard to identify a correct or incorrect empty data response.