Fix reading dicom series error in ITKReader

[KumoLiu]

Fixes #6942
fixes #6944.

Description

When using itk backend reading dicom series and there is only one file in the folder, return the first name since itk assuming that there must be more than one file in the tuple.
https://github.com/InsightSoftwareConsortium/ITK/blob/452cb3f9716e95532200de3196d51cd705a0ec5e/Wrapping/Generators/Python/itk/support/extras.py#L1275-L1284

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.

[wyli]

Thanks, it looks good to me.

[wyli]

/build