fix: upload 0-byte object when closing open_s3_object in write mode

[hsusul]

Description

Fixes #3421

Affected API

awswrangler.s3._fs.open_s3_object / awswrangler.s3

Background & Root Cause

When an S3 object stream is opened in write mode (mode="w" or mode="wb") using open_s3_object, and closed after writing 0 bytes (e.g. creating an empty file, writing b"", or exiting the context manager without write()), _S3ObjectBase.close() previously checked:

if self.writable():
    if self._parts_count > 0:
        ... # complete multipart upload
    elif self._buffer.tell() > 0:
        ... # put_object

When self._parts_count == 0 and self._buffer.tell() == 0 (0 bytes written), both conditions evaluated to False. Consequently, put_object was skipped, leaving the S3 object non-existent if new, or retaining pre-existing content if overwriting.

Corrected Behavior

_S3ObjectBase.close() now calls put_object whenever self._parts_count == 0, uploading Body=self._buffer.getvalue() (which is b"" when 0 bytes are written). This matches standard Python file semantics (open(path, "wb")), correctly creating or truncating the S3 object to 0 bytes.

Local Reproduction & Verification

Added a unit test test_open_s3_object_empty_file_write in tests/unit/test_moto.py covering:

1. Opening a non-existent S3 path in "wb" mode without writing, verifying does_object_exist returns True and ContentLength == 0.
2. Opening an existing S3 path containing prior data in "w" mode and writing "", verifying the object is truncated to 0 bytes (ContentLength == 0).

Exact Validation Results

• pytest tests/unit/test_moto.py -> 46 passed in 3.25s
• pytest tests/unit/test_utils.py -> 18 passed in 0.02s
• ruff format --check . -> Passed
• ruff check . -> Passed
• git diff --check -> Clean

AWS Integration Tests Not Run

Live AWS service tests (requiring AWS infrastructure / credentials) were not executed. Verification was completed using the repository's unit test suite with moto.

Compatibility & Resource Ownership Behavior

• Preserves all public APIs and existing return types.
• Idempotent stream closure logic remains intact.
• No third-party dependencies added or mutated.