TST: add test to confirm GH14606 (specify category dtype for empty)

Issue pandas-dev#14606 was fixed by PR pandas-dev#14717, adding one more specific test to confirm this

Author: jorisvandenbossche

pandas/io/tests/parser/dtypes.py

@@ -241,6 +241,9 @@ def test_empty_dtype(self):
         result = self.read_csv(StringIO(data), header=0,
                                dtype='category')
         tm.assert_frame_equal(result, expected)
+        result = self.read_csv(StringIO(data), header=0,
+                               dtype={'a': 'category', 'b': 'category')
+        tm.assert_frame_equal(result, expected)
 
         expected = pd.DataFrame(columns=['a', 'b'], dtype='datetime64[ns]')
         result = self.read_csv(StringIO(data), header=0,