Tar timestamp handling followup

[carlossanlop]

Creating this issue to further discuss and track the additional feedback received in #115211 :

• Handle mtime the same way as atime and ctime, where if the value is default, it would have to be written with all nulls: Tar: Refine GNU timestamp handling #115211 (comment)

• PAX probably shouldn't add atime and ctime to the ExtendedAttributes dictionary by default: Tar: Refine GNU timestamp handling #115211 (comment)

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