14 to 15 does not work with empty `HTTPHeader` field

[Jorropo]

With "HTTPHeaders": {}, I get this:

> ipfs daemon
Initializing daemon...
Kubo version: 0.23.0-dev
Repo version: 15
System version: amd64/linux
Golang version: go1.21.0
Found outdated fs-repo, migrations need to be run.
Run migrations now? [y/N] y
Looking for suitable migration binaries.
Need 1 migrations, downloading.
Downloading migration: fs-repo-14-to-15...
Fetching with HTTP: "https://ipfs.io/ipfs/QmVYvjxCiUxqGjDeREFDy8SbX3zj7vhgeuprk28hgvKEq5/fs-repo-14-to-15/versions"
Fetching with HTTP: "https://ipfs.io/ipfs/QmVYvjxCiUxqGjDeREFDy8SbX3zj7vhgeuprk28hgvKEq5/fs-repo-14-to-15/v1.0.0/fs-repo-14-to-15_v1.0.0_linux-amd64.tar.gz"
Fetching with HTTP: "https://ipfs.io/ipfs/QmVYvjxCiUxqGjDeREFDy8SbX3zj7vhgeuprk28hgvKEq5/fs-repo-14-to-15/v1.0.0/fs-repo-14-to-15_v1.0.0_linux-amd64.tar.gz"
Fetching with HTTP: "https://ipfs.io/ipfs/QmVYvjxCiUxqGjDeREFDy8SbX3zj7vhgeuprk28hgvKEq5/fs-repo-14-to-15/v1.0.0/fs-repo-14-to-15_v1.0.0_linux-amd64.tar.gz"
Downloaded and unpacked migration: /tmp/migrations1481320887/fs-repo-14-to-15 (v1.0.0)
Running migration fs-repo-14-to-15 ...
  => Running: /tmp/migrations1481320887/fs-repo-14-to-15 -path=/home/hugo/.ipfs -verbose=true
applying 14-to-15 repo migration
locking repo at "/home/hugo/.ipfs"
  - verifying version is '14'
> Upgrading config to new format
error: invalid type for .Gateway.HTTPHeaders[Access-Control-Allow-Origin] got <nil> expected json array
The migrations of fs-repo failed:
  migration fs-repo-14-to-15 failed: exit status 1
If you think this is a bug, please file an issue and include this whole log output.
  https://github.com/ipfs/fs-repo-migrations

Error: migration fs-repo-14-to-15 failed: exit status 1