fix: avoid black images if using an invalid VAE (for SDXL) #1954

Job	Run time
ubuntu-latest-cmake-vulkan	5m 53s
ubuntu-latest-cmake	3m 6s
ubuntu-latest-rocm	39m 45s
macOS-latest-cmake	4m 39s
windows-latest-cmake-hip	2m 14s
Build and push container images (vulkan)	6m 57s
Build and push container images (musa)	45m 43s
Build and push container images (sycl)	9m 3s
windows-latest-cmake (noavx, -DGGML_NATIVE=OFF -DGGML_AVX=OFF -DGGML_AVX2=OFF -DGGML_FMA=OFF -DSD...	1m 43s
windows-latest-cmake (vulkan, -DSD_VULKAN=ON -DSD_BUILD_SHARED_LIBS=ON)	5m 5s
windows-latest-cmake (avx2, -DGGML_NATIVE=OFF -DGGML_AVX2=ON -DSD_BUILD_SHARED_LIBS=ON)	2m 40s
windows-latest-cmake (avx, -DGGML_NATIVE=OFF -DGGML_AVX=ON -DGGML_AVX2=OFF -DSD_BUILD_SHARED_LIBS...	2m 13s
windows-latest-cmake (cuda12, -DSD_CUDA=ON -DSD_BUILD_SHARED_LIBS=ON -DCMAKE_CUDA_ARCHITECTURES='...	2h 3m 39s
windows-latest-cmake (avx512, -DGGML_NATIVE=OFF -DGGML_AVX512=ON -DGGML_AVX=ON -DGGML_AVX2=ON -DS...	1m 45s
release	0s
	4h 14m 25s

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