Do not export an empty to path to a windows image config #2

Job	Run time
prepare	4s
binaries (darwin/amd64)	1m 16s
binaries (darwin/arm64)	1m 2s
binaries (linux/amd64)	2m 6s
binaries (linux/arm/v7)	2m 12s
binaries (linux/arm64)	2m 17s
binaries (linux/s390x)	1m 58s
binaries (linux/ppc64le)	2m 2s
binaries (linux/riscv64)	2m 16s
binaries (windows/amd64)	1m 43s
binaries (windows/arm64)	1m 46s
test / prepare	5m 19s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 18s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 42s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 40s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 8s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	4m 59s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 4s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 20s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 8s
test / run (./..., 1, integration gateway)	4m 11s
test / run (./client, containerd, nydus, integration)	7m 7s
test / run (./client, oci, nydus, integration)	4m 51s
test / run (./..., nydus, 1, integration)	4m 12s
image	5m 42s
image (rootless)	5m 42s
release	1s
	1h 48m 6s

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