TST: Update union_categorical tests

This relied on `NaN` being a float and empty being a float. Not a necessary
test anymore.

pandas/tests/reshape/test_union_categoricals.py

@@ -107,17 +107,11 @@ def test_union_categoricals_empty(self):
         exp = Categorical([])
         tm.assert_categorical_equal(res, exp)
 
-        res = union_categoricals([pd.Categorical([]),
-                                  pd.Categorical([1.0])])
-        exp = Categorical([1.0])
+        res = union_categoricals([Categorical([]),
+                                  Categorical(['1'])])
+        exp = Categorical(['1'])
         tm.assert_categorical_equal(res, exp)
 
-        # to make dtype equal
-        nanc = pd.Categorical(np.array([np.nan], dtype=np.float64))
-        res = union_categoricals([nanc,
-                                  pd.Categorical([])])
-        tm.assert_categorical_equal(res, nanc)
-
     def test_union_categorical_same_category(self):
         # check fastpath
         c1 = Categorical([1, 2, 3, 4], categories=[1, 2, 3, 4])