fix preprocessor test

testing/preprocessor_test.py

@@ -43,7 +43,8 @@ def list_tifs(products):
     return preprocessed_list
 
 def test_preprocessor(list_tifs):
-    
+    # check if there are preprocessed results in the buckets 
+    assert len(list_tifs) > 0
     for item in list_tifs:
         set_gdal_swift_auth()
         image = gdal.Open('/vsiswift/preprocessor_results/%s' % item, gdal.GA_ReadOnly)