BazaRb::Fake#durable_save rejects empty files that the real client uploads

[gemshrine]

BazaRb::Fake#durable_save rejects an existing zero-byte file, while the real BazaRb#durable_save uploads it. This makes tests with the fake reject a valid client call.

Steps to reproduce

1. Create an empty file with File.write(path, '').
2. Call BazaRb::Fake.new.durable_save(1, path).

Actual result: BazaRb::Fake#checkfile raises The file must be non-empty because it requires File.size(file).positive?.

Expected result: the fake should accept the same file input as the real BazaRb#durable_save, whose validation only checks that the file exists before uploading it.

The mismatch is between lib/baza-rb/fake.rb:checkfile and lib/baza-rb.rb:durable_save. It matters when a durable intentionally represents an empty artifact.

[gemshrine]

Duplicate of #449. Both reports come from the same BazaRb::Fake#checkfile size check, which rejects an empty file even though the real client accepts it. The fix tracked in #449 should cover durable_save as well as durable_place.