gh-149578: Fix tarfile.open failing on PAX archives with only global headers

[ShadiBahaa]

Summary

Fix tarfile.open() raising ReadError when opening a PAX format tar archive that contains only global headers and no regular file members.

Root cause: In TarInfo._proc_pax(), after processing a global header (XGLTYPE), the code tries to read the next header. If the archive has no file entries after the global header, this read encounters the end-of-archive marker and raises EOFHeaderError. This exception was being caught by the generic except HeaderError clause and converted to SubsequentHeaderError, which both TarFile.next() and the append-mode initialization loop treat as a fatal error.

Fix: Catch EOFHeaderError separately before the generic HeaderError handler. For global headers (XGLTYPE), let the EOFHeaderError propagate so callers handle end-of-archive normally. For extended headers (XHDTYPE), a following file entry is mandatory, so the error is still converted to SubsequentHeaderError.

Test plan

• Added test_pax_global_header_empty_archive in PaxWriteTest that:
  • Creates a PAX archive with global headers but no file entries
  • Verifies the archive can be opened for reading and global headers are preserved
  • Verifies the archive can be opened in append mode
  • Verifies appending a file works and global headers are preserved afterward

• Issue: tarfile: Failed opening empty PAX TAR with global headers #149578

[python-cla-bot]

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