Add dtype of expected result in testcase

Author: chilin0525

Diff for: pandas/tests/indexes/period/test_setops.py

@@ -372,5 +372,5 @@ def test_difference_periodindex_with_index(self):
         tm.assert_index_equal(result1, expected1)
 
         result2 = index2.difference(index1)
-        expected2 = pd.Index([])
+        expected2 = pd.Index([], dtype=index2.dtype)
         tm.assert_index_equal(result2, expected2)