utils: get_compacted_slices: fix issue if urls are empty

nabu/io/utils.py

@@ -46,6 +46,9 @@ def get_compacted_dataslices(urls, subsampling=None):
     def merge_slices(slice1, slice2):
         return slice(slice1.start, slice2.stop)
 
+    if len(urls) == 0:
+        return urls
+
     sorted_files_indices = sorted(urls.keys())
     idx0 = sorted_files_indices[0]
     first_url = urls[idx0]