Fix ICO save producing an empty file for images smaller than 16x16

[gaoflow]

Saving an image smaller than the smallest default ICO size (16×16) as ICO, without an explicit sizes argument, silently produces an empty, unreadable file:

import io
from PIL import Image

buf = io.BytesIO()
Image.new("RGBA", (8, 8), (10, 20, 30, 255)).save(buf, "ICO")
print(len(buf.getvalue()))          # 6  -> just the ICONDIR header, idCount=0
buf.seek(0)
Image.open(buf).load()              # UnidentifiedImageError: cannot identify image file

The 8×8 image is representable as an ICO (save(buf, "ICO", sizes=[(8, 8)]) works fine), but the default-sizes path drops it entirely with no error or warning.

Cause

The default sizes start at (16, 16), and _save skips every candidate size larger than the source image. For an image smaller than 16×16 every default candidate is skipped, so frames stays empty and o16(len(frames)) writes idCount=0 — a 6-byte header-only file.

Fix

Filter the requested sizes up front, and if none fit, fall back to the image's own size (capped at the 256×256 ICO maximum) so a valid, readable icon is always written. This also fixes the related case where every explicitly requested size is larger than the image (previously also an empty file) — consistent with the documented behaviour that oversized requested sizes are ignored.

Tests

test_save_smaller_than_default_sizes parametrizes (1, 1), (8, 8), (15, 15) and the non-square (8, 12), asserting each round-trips to a valid ICO of the right size with info["sizes"] and a corner pixel preserved. test_save_all_sizes_larger_than_image pins the explicit-sizes fallback. All fail on main with UnidentifiedImageError and pass with the fix; the full Tests/test_file_ico.py suite stays green (27 passed); ruff format, ruff check and mypy are clean.

[codspeed-hq]

Merging this PR will not alter performance

✅ 331 untouched benchmarks

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_{Comparing gaoflow:fix-ico-save-small-image-empty (5bb1b35) with main (6590b1b)}