Optimize sampling for empty images #1616

Job	Run time
Fuzz targets (afl)	2m 36s
build_benchmarks	45s
Fuzz targets (cargo-fuzz)	54s
cargo-deny	20s
cargo-semver-checks	27s
clippy	2m 3s
public_private_dependencies	26s
rustfmt	7s
test_avif_decoding	1m 18s
test_features	22s
test_other_archs (powerpc-unknown-linux-gnu)	2m 27s
test_features (default)	1m 53s
test_toolchains (1.67.1)	46s
test_other_archs (i686-unknown-linux-gnu)	1m 35s
test_features (rayon)	32s
test_toolchains (nightly)	1m 10s
test_features (avif)	1m 1s
test_toolchains (beta)	1m 19s
test_features (bmp)	24s
test_features (dds)	23s
test_features (exr)	47s
test_features (ff)	20s
test_features (gif)	29s
test_features (hdr)	24s
test_features (ico)	34s
test_features (jpeg)	25s
test_features (png)	28s
test_features (pnm)	26s
test_features (qoi)	28s
test_features (tga)	25s
test_features (tiff)	53s
test_features (webp)	28s
	26m 55s

Provide feedback