tarfile.data_filter doesn't handle symlinks with empty names

[encukou]

tarfile.data_filter can be bypassed using a chain of empty-named symlinks (name="").

Linked PRs

• gh-149486: tarfile.data_filter: validate written link target #149487
• [3.15] gh-149486: tarfile.data_filter: validate written link target (GH-149487) #149553
• [3.14] gh-149486: tarfile.data_filter: validate written link target (GH-149487) #149554
• [3.13] gh-149486: tarfile.data_filter: validate written link target (GH-149487) #149555
• [3.12] gh-149486: tarfile.data_filter: validate written link target (GH-149487) #149556