fix: Write one zero offset for empty variable-size arrays

[rustyconover]

An offsets buffer holds length + 1 offsets, so even an empty string, binary, list or map array has one: offsets[0] = 0. The array view of an empty array has a zero-length offsets buffer, though (ArrowArrayViewSetArray() deliberately doesn't require one), and ArrowIpcEncoderCollectArray() wrote that buffer as is. So every empty variable-size array nanoarrow writes has a 0-byte offsets buffer.

IPC permits that, and Arrow C++ reads it, but not every reader does. With nanoarrow 0.9.0 (Python) writing struct<s: string, l: list<int32>> with zero rows, arrow-go v18.8.0 reads the batch and then panics on String.ValueOffsets():

panic: runtime error: slice bounds out of range [:1] with capacity 0

The same batch written by pyarrow 25.0.1 has a 4-byte offsets buffer (8 bytes for large_string), and arrow-go reads it. Apache.Arrow for .NET had the same problem (apache/arrow-dotnet#462, now fixed there, but released versions still throw).

The encoder now writes one zero offset (4 or 8 bytes) in place of a zero-length offsets buffer of an empty array, as Arrow C++ does. Non-empty arrays and other buffer types are unchanged.

Tests:

• NanoarrowIpcEncoderWritesOffsetForEmptyArrays encodes an empty struct<s: string, b: binary, ls: large_string, l: list<int32>> built the usual way. It decodes the message and checks that each child's offsets buffer holds one zero offset of the right width. On main the test fails: the buffers are 0 bytes.
• AssertArrayViewIdentical(), used by the decoder round-trip tests, compared buffer sizes exactly. It now expects that one zero offset where the original empty array's view has a zero-length offsets buffer.

ctest passes (Linux aarch64, Debug, 868 tests with NANOARROW_BUILD_TESTS_WITH_ARROW), and 393 tests under ASan/UBSan with clang. The changed files are formatted with clang-format 16.0.6, the pre-commit version.

This PR's code and description were written by AI (Claude Code) and reviewed by a human maintainer.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.30%. Comparing base (10885c5) to head (f23b5c8).

Files with missing lines	Patch %	Lines
src/nanoarrow/ipc/encoder.c	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #978   +/-   ##
=======================================
  Coverage   78.30%   78.30%           
=======================================
  Files         106      106           
  Lines       16919    16924    +5     
  Branches     2016     2018    +2     
=======================================
+ Hits        13248    13252    +4     
  Misses       2429     2429           
- Partials     1242     1243    +1     

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[paleolimbot]

Thank you!