fix(dataframe): claim remaining restriction range on empty/header-only CSV reads

[ManvithPanyam]

This is my first contribution to Apache Beam — happy to adjust the approach if there's a preferred pattern I'm missing.

What broke?

When reading an empty or header-only CSV file using beam.io.ReadFromCsv, the pipeline fails with:

ValueError: OffsetRestrictionTracker is not done since work in range [0, 16) has not been claimed.

Root Cause

In sdks/python/apache_beam/dataframe/io.py, _TruncatingFileHandle._read() returns early on an empty buffer without claiming the remaining restriction range, causing check_done() to fail.

Fix

if not self._buffer:
  self._tracker.try_claim(self._tracker.current_restriction().stop)
  self._done = True
  return self._empty

Verification

Added test_empty_csv_read to IOTest. Full suite (36 tests) passes with the fix; fails without it. Pre-existing Windows-only failures are unrelated (#20642).

[gemini-code-assist]

Caution

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[github-actions]

Assigning reviewers:

R: @tvalentyn for label python.

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