add a couple of tests

Author: dimitri-yatsenko

tests/test_fetch.py

@@ -213,3 +213,5 @@ def test_nullable_numbers(self):
         table.insert1((100, None, None, None))
         f, d, i = table.fetch('fvalue', 'dvalue', 'ivalue')
         assert_true(None in i)
+        assert_true(any(np.isnan(d)))
+        assert_true(any(np.isnan(f)))