Optimize sampling for empty images #1619

Job	Run time
Fuzz targets (cargo-fuzz)	1m 0s
Fuzz targets (afl)	2m 43s
cargo-deny	21s
build_benchmarks	48s
cargo-semver-checks	24s
clippy	2m 2s
public_private_dependencies	23s
rustfmt	6s
test_avif_decoding	1m 30s
test_features	19s
test_other_archs (powerpc-unknown-linux-gnu)	2m 25s
test_features (default)	1m 44s
test_toolchains (1.67.1)	44s
test_other_archs (i686-unknown-linux-gnu)	1m 15s
test_features (rayon)	21s
test_toolchains (nightly)	1m 43s
test_features (avif)	54s
test_toolchains (beta)	1m 24s
test_features (bmp)	19s
test_features (dds)	21s
test_features (exr)	36s
test_features (ff)	18s
test_features (gif)	23s
test_features (hdr)	17s
test_features (ico)	32s
test_features (jpeg)	20s
test_features (png)	22s
test_features (pnm)	17s
test_features (qoi)	17s
test_features (tga)	18s
test_features (tiff)	35s
test_features (webp)	23s
	25m 24s

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