BUG: chokes on pd.DatetimeTZDtype if there are no rows. (#59123)

BUG: `pivot_table` chokes on pd.DatetimeTZDtype if there are no rows.

This is a follow up to #41875

Author: pcorpet

doc/source/whatsnew/v3.0.0.rst

@@ -596,6 +596,7 @@ Reshaping
 ^^^^^^^^^
 - Bug in :meth:`DataFrame.join` inconsistently setting result index name (:issue:`55815`)
 - Bug in :meth:`DataFrame.unstack` producing incorrect results when ``sort=False`` (:issue:`54987`, :issue:`55516`)
+- Bug in :meth:`DataFrame.unstack` producing incorrect results when manipulating empty :class:`DataFrame` with an :class:`ExtentionDtype` (:issue:`59123`)
 
 Sparse
 ^^^^^^

pandas/core/reshape/reshape.py

@@ -288,21 +288,19 @@ def get_new_values(self, values, fill_value=None):
 
         dtype = values.dtype
 
-        # if our mask is all True, then we can use our existing dtype
-        if mask_all:
-            dtype = values.dtype
-            new_values = np.empty(result_shape, dtype=dtype)
-        else:
-            if isinstance(dtype, ExtensionDtype):
-                # GH#41875
-                # We are assuming that fill_value can be held by this dtype,
-                #  unlike the non-EA case that promotes.
-                cls = dtype.construct_array_type()
-                new_values = cls._empty(result_shape, dtype=dtype)
+        if isinstance(dtype, ExtensionDtype):
+            # GH#41875
+            # We are assuming that fill_value can be held by this dtype,
+            #  unlike the non-EA case that promotes.
+            cls = dtype.construct_array_type()
+            new_values = cls._empty(result_shape, dtype=dtype)
+            if not mask_all:
                 new_values[:] = fill_value
-            else:
+        else:
+            if not mask_all:
                 dtype, fill_value = maybe_promote(dtype, fill_value)
-                new_values = np.empty(result_shape, dtype=dtype)
+            new_values = np.empty(result_shape, dtype=dtype)
+            if not mask_all:
                 new_values.fill(fill_value)
 
         name = dtype.name

pandas/tests/reshape/test_pivot.py

@@ -2769,3 +2769,17 @@ def test_unstack_copy(self, m):
         result = df.unstack(sort=False)
         result.iloc[0, 0] = -1
         tm.assert_frame_equal(df, df_orig)
+
+    def test_pivot_empty_with_datetime(self):
+        # GH#59126
+        df = DataFrame(
+            {
+                "timestamp": Series([], dtype=pd.DatetimeTZDtype(tz="UTC")),
+                "category": Series([], dtype=str),
+                "value": Series([], dtype=str),
+            }
+        )
+        df_pivoted = df.pivot_table(
+            index="category", columns="value", values="timestamp"
+        )
+        assert df_pivoted.empty