include @amueller s suggestion

Author: mfeurer

openml/datasets/dataset.py

@@ -460,9 +460,10 @@ def _check_qualities(qualities):
         qualities_ = {}
         for xmlquality in qualities:
             name = xmlquality['oml:name']
-            if 'oml:value' not in xmlquality:
+
+            if xmlquality.get('oml:value', None) is None:
                 value = float('NaN')
-            elif xmlquality['oml:value'] is None:
+            elif 'oml:value' not in xmlquality:
                 value = float('NaN')
             elif xmlquality['oml:value'] == 'null':
                 value = float('NaN')