Return "data not available" for sites that don't have data

If data is requested for a site outside of the calculation region (but possibly still inside the calculation bounds [min/max lon lat]), the ys values will all be null. Instead of a full response with null hazard curves, detect this and return a response to indicate that data is not available for the requested site. For example: https://earthquake.usgs.gov/ws/nshmp/conus-2018/static/hazard/-82.5/42.1

This should also occur for sites that may be along the edge of the calculation region that do not have valid data at all four bounding sites (snapped to the grid).