fix empty segment case in data availability check

locklost/data.py

@@ -193,7 +193,10 @@ def data_available(segment):
         segs = gwdatafind.find_times(
             ifo, ftype, start, end,
         )
-        return segment[1] >= segs.extent()[1]
+        seg_available = segs.extent()
+        if not seg_available:
+            return False
+        return segment[1] >= seg_available[1]
     else:
         raise ValueError("unknown data access method: {}".format(DATA_ACCESS))