history: fix empty Exporters attribute #3781

Job	Run time
prepare	5s
binaries (darwin/amd64)	58s
binaries (darwin/arm64)	1m 10s
binaries (linux/amd64)	2m 1s
binaries (linux/arm/v7)	1m 57s
binaries (linux/arm64)	2m 19s
binaries (linux/s390x)	1m 57s
binaries (linux/ppc64le)	2m 2s
binaries (linux/riscv64)	2m 3s
binaries (windows/amd64)	1m 48s
binaries (windows/arm64)	1m 36s
test / prepare	1m 51s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	8m 2s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	9m 14s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 31s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 22s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 25s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 15s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 21s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 23s
test / run (./..., 1, integration gateway)	4m 15s
test / run (./client, containerd, nydus, integration)	7m 30s
test / run (./client, oci, nydus, integration)	5m 17s
test / run (./..., nydus, 1, integration)	4m 9s
image	0s
release	0s
	1h 35m 31s

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