When xuint URL is None, log summary and overall result.

[kwk]

Lately I've been seeing these errors:

[05/Aug/2025 22:34:42] INFO [util.py:137 read_url_response_into_file] Getting URL https://api.testing-farm.io/v0.1/requests/985cbe62-61dc-48c9-9428-0603509cf570
[05/Aug/2025 22:34:42] INFO [tfutil.py:236 get_xunit_file_from_request_file] Fetching xunit URL from URL: None
[05/Aug/2025 22:34:42] INFO [util.py:137 read_url_response_into_file] Getting URL None
Traceback (most recent call last):
  File "/home/runner/work/llvm-snapshots/llvm-snapshots/snapshot_manager/main.py", line 446, in <module>
    main()
  File "/home/runner/work/llvm-snapshots/llvm-snapshots/snapshot_manager/main.py", line 80, in main
    SnapshotManager(config=cfg).check_todays_builds()
  File "/home/runner/work/llvm-snapshots/llvm-snapshots/snapshot_manager/snapshot_manager/snapshot_manager.py", line 499, in check_todays_builds
    request.fetch_failed_test_cases(
  File "/home/runner/work/llvm-snapshots/llvm-snapshots/snapshot_manager/testing_farm/request.py", line 168, in fetch_failed_test_cases
    xunit_file = tfutil.get_xunit_file_from_request_file(
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/llvm-snapshots/llvm-snapshots/snapshot_manager/testing_farm/tfutil.py", line 239, in get_xunit_file_from_request_file
    xuint_file = util.read_url_response_into_file(xunit_url)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/llvm-snapshots/llvm-snapshots/snapshot_manager/snapshot_manager/util.py", line 138, in read_url_response_into_file
    response = requests.get(url)
               ^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/requests/api.py", line 73, in get
    return request("get", url, params=params, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/requests/api.py", line 59, in request
    return session.request(method=method, url=url, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/requests/sessions.py", line 575, in request
    prep = self.prepare_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/requests/sessions.py", line 484, in prepare_request
    p.prepare(
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/requests/models.py", line 367, in prepare
    self.prepare_url(url, params)
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/requests/models.py", line 438, in prepare_url
    raise MissingSchema(
requests.exceptions.MissingSchema: Invalid URL 'None': No scheme supplied. Perhaps you meant https://None?

This is the relevant part of the JSON.

  "result": {
    "summary": "Failed to discover tests, TMT metadata are absent or corrupted. Search for '[E] [test-schedule-tmt' on the artifacts page for details.",
    "overall": "error",
    "xunit_url": null
  },

With this change, we'll stop trying to fetch the xuint_url if it is None and instead log the error.