Merge pull request #1958 from ceb8/fix_mast_tests

Fixing PR 1921

Author: C. E. Brasseur

astroquery/mast/observations.py

@@ -427,17 +427,19 @@ def get_product_list_async(self, observations):
         -------
             response : list of `~requests.Response`
         """
-        if len(observations) == 0 or not observations["obsid"]:
-            raise InvalidQueryError("Observation list is empty, no associated products.")
 
         # getting the obsid list
         if isinstance(observations, Row):
             observations = observations["obsid"]
         if np.isscalar(observations):
-            observations = [observations]
+            observations = np.array([observations])
         if isinstance(observations, Table):
             observations = observations['obsid']
 
+        observations = observations[observations != ""]
+        if len(observations) == 0:
+            raise InvalidQueryError("Observation list is empty, no associated products.")
+
         service = 'Mast.Caom.Products'
         params = {'obsid': ','.join(observations)}