gh-159354: Validate explicit main arguments in zipapp

[esadomer]

create_archive() silently ignores main when copying an existing archive. Reject any value other than None before copying so invalid calls preserve their destination.

Apply the same distinction when creating an archive from a directory: an empty main is an explicit, invalid entry point, including when __main__.py already exists. The CLI also rejects an empty --main when copying without producing a traceback.

Validation (Windows debug build): test_zipapp passed (36 tests run, 2 skipped). The two directory cases and CLI case failed before the corresponding fixes. Coverage includes path and file-object archive sources, valid/default entry points, and destination preservation. Ruff and git diff --check passed.

Fixes #159354.

• Issue: Reject main when copying an existing zipapp archive #159354