fix DownloadPipeline.fetch_many() crashing on a missing chunk

[ThomasWaldmann]

Split out of #10023 (where this was found), because it is an independent bug fix.

With replacement_chunk=False, fetch_many() is documented (and used) to yield None for a
chunk that is missing in the repository - but the size check right before the yield then did
len(None) and raised TypeError instead.

borg webdav is currently the only caller that passes replacement_chunk=False for file
content, so its "chunk missing" path (abort the connection instead of serving corrupted data)
never actually ran: the TypeError ended up in the request error handler, which then tried to
send a 500 for a response whose headers were already on the wire.

It shows up as a test failure in #10023 because borg mount reads via fetch_many() there,
so test_fuse_allow_damaged_files reported EINVAL instead of EIO on all three FUSE legs.

Tests: unit tests for both flavours of a missing chunk (the replacement_chunk=False one fails
without this fix) and an end-to-end webdav test that deletes a chunk and expects the download
to be aborted rather than completed.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.14%. Comparing base (4d2e068) to head (d5eb3f7).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10024      +/-   ##
==========================================
+ Coverage   86.10%   86.14%   +0.04%     
==========================================
  Files          96       96              
  Lines       17326    17326              
  Branches     2649     2649              
==========================================
+ Hits        14918    14925       +7     
+ Misses       1667     1663       -4     
+ Partials      741      738       -3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.