Do not invoke bulk_chunked for zero shape

[fallintoplace]

What changed

bulk_chunked currently calls the function once with (0, 0) when the shape is zero. exec.bulk requires each chunk to satisfy b < e, so an empty shape has no valid chunk.

• Skip the callback when shape <= 0.
• Still forward the value completion.
• Add regressions for zero and signed negative shapes.

The exposition-only default implementation in the same section currently calls f(0, shape, args...) unconditionally. That looks inconsistent with the non-empty chunk requirement. This change follows the behavioral requirement.

Test

• build/test/test.stdexec "[adaptors][bulk]" (363 assertions, 59 test cases)
• ctest --test-dir build --output-on-failure (953 passed)

See exec.bulk.

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[ericniebler]

/ok to test ffcb102

[ericniebler]

/ok to test ef2bf69