[8.8.0] Preserve empty `factsVersions` map in the lockfile merge driver

[bazel-io]

Forked from #29748

[bazel-io]

Cherry-pick was attempted but there were merge conflicts in the following file(s). Please resolve manually.

scripts/bazel-lockfile-merge.jq
scripts/bazel_lockfile_merge_test.sh

cc: @bazelbuild/triage

[iancha1992]

Cherry-picked in #29780

[iancha1992]

@fmeum I am going to mark this as a soft-release-blocker