GNSS: handle empty return

[webgisdeveloper]

test case: python getCoseismic.py --lat 32.13 --lon -115.30 --width 1 --height 1 -o coseismic.kml -t 2010-04-04

return:
more coseismic.kml_table.txt
Site Lon Lat Delta E Delta N Delta V Sigma E Sigma N Sigma V