…) for empty timestamp frames

### What changes were proposed in this pull request?

This PR updates pandas-on-Spark `DataFrame.describe()` and the related `test_describe_empty` expectations for empty timestamp-containing frames to handle the pandas 3 `astype(str)` behavior change on null values.

In pandas 2, empty timestamp stats were string-converted as `"None"` in the relevant `describe()` path. In pandas 3, `astype(str)` preserves those empty stats as missing values instead. This patch updates the pandas-on-Spark result construction and the corresponding test expectations to follow that behavior consistently.

### Why are the changes needed?

`pyspark.pandas.tests.computation.test_describe FrameDescribeTests.test_describe_empty` fails with pandas 3 because pandas changed how `astype(str)` handles null values in empty timestamp `describe()` results.

Without this change, pandas-on-Spark and the pandas-based expectation disagree for empty timestamp-only and mixed timestamp frames.

### Does this PR introduce _any_ user-facing change?

Yes.

For pandas-on-Spark `DataFrame.describe()` on empty timestamp-containing frames, null timestamp stats now follow the pandas 3 string-conversion behavior instead of always being materialized as `"None"`.

### How was this patch tested?

Ran the related `pyspark.pandas.tests.computation.test_describe` tests in both pandas 2 and pandas 3 Python environments.

### Was this patch authored or co-authored using generative AI tooling?

Generated-by: OpenAI Codex (GPT-5)

Closes apache#54893 from ueshin/issues/SPARK-56060/describe.

Authored-by: Takuya Ueshin <ueshin@databricks.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>