history: fix empty Exporters attribute #3784

Job	Run time
prepare	5s
binaries (darwin/amd64)	59s
binaries (darwin/arm64)	1m 45s
binaries (linux/amd64)	2m 14s
binaries (linux/arm/v7)	2m 20s
binaries (linux/arm64)	2m 7s
binaries (linux/s390x)	1m 59s
binaries (linux/ppc64le)	1m 56s
binaries (linux/riscv64)	2m 4s
binaries (windows/amd64)	1m 38s
binaries (windows/arm64)	1m 39s
test / prepare	3m 58s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 20s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	9m 18s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	7m 19s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 18s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	5m 44s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	8m 27s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	5m 43s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	5m 15s
test / run (./..., 1, integration gateway)	4m 55s
test / run (./client, containerd, nydus, integration)	7m 30s
test / run (./client, oci, nydus, integration)	5m 37s
test / run (./..., nydus, 1, integration)	4m 31s
image	8m 53s
image (rootless)	8m 45s
release	14s
	2h 0m 33s

Provide feedback