[BUG] beacons.save does not save beacons defined in pillar data

Description
According to the documentation here, beacons defined in pillars are delivered to the minion in memory, but can be saved to the filesystem using beacons.save.

I have a simple diskusage beacon applied in pillar data, but it is not persisted when using beacons.save.

Setup

# Salt master config
> cat /etc/salt/master.d/pillar.conf
pillar_roots:
  sandbox:
    - /srv/pillar/sandbox

# Pillar file
> cat /srv/pillar/sandbox/beacons/init.sls
beacons:
  diskusage:
    - /: 11%
    - interval: 60

# Top file
> cat /srv/pillar/sandbox/top.sls
sandbox:
  'diskusage_beacon_sandbox':
    - beacons

With that configured, I apply my pillars and validate, then use beacons.save to persist it -- the output from beacons.save would indicate that the beacon persisted, but the file it creates is empty.

# Apply pillars
> salt -L diskusage_beacon_sandbox saltutil.pillar_refresh
diskusage_beacon_sandbox:
    True

# Validate
> salt -L diskusage_beacon_sandbox beacons.list
diskusage_beacon_sandbox:
    beacons:
      diskusage:
      - /: 11%
      - interval: 60

# Save beacons
> salt -L diskusage_beacon_sandbox beacons.save
diskusage_beacon_sandbox:
    ----------
    comment:
        Beacons saved to /etc/salt/minion.d/beacons.conf.
    result:
        True

# Check beacons.conf on the minion -- the file is created but it is empty
> salt -L diskusage_beacon_sandbox file.read /etc/salt/minion.d/beacons.conf
diskusage_beacon_sandbox:

# File size shows 0 bytes
> salt -L diskusage_beacon_sandbox cmd.run 'ls -lh /etc/salt/minion.d/beacons.conf'
diskusage_beacon_sandbox:
    -rw-r--r--. 1 root root 0 Jun  2 18:42 /etc/salt/minion.d/beacons.conf

I am testing this using a RHEL8 EC2 instance that I provisioned with salt-cloud.

Expected behavior
I would expect the beacon data to be persisted in /etc/salt/minion.d/beacons.conf. Note that manually deploying the beacon by landing a .conf file in /etc/salt/minion.d works as does using beacons.add.

Versions Report

salt --versions-report
Salt Version:
          Salt: 3006.9

Python Version:
        Python: 3.10.14 (main, Jun 26 2024, 11:44:37) [GCC 11.2.0]

Dependency Versions:
          cffi: 1.17.1
      cherrypy: unknown
  cryptography: 42.0.5
      dateutil: 2.8.1
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 3.1.4
       libgit2: 1.7.2
  looseversion: 1.0.2
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.2
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     packaging: 22.0
     pycparser: 2.21
      pycrypto: Not Installed
  pycryptodome: 3.19.1
        pygit2: 1.14.1
  python-gnupg: 0.4.8
        PyYAML: 6.0.1
         PyZMQ: 23.2.0
        relenv: 0.17.0
         smmap: Not Installed
       timelib: 0.2.4
       Tornado: 4.5.3
           ZMQ: 4.3.4

System Versions:
          dist: rhel 8.10 Ootpa
        locale: utf-8
       machine: x86_64
       release: 4.18.0-553.16.1.el8_10.x86_64
        system: Linux
       version: Red Hat Enterprise Linux 8.10 Ootpa

My minion is also running salt-minion-3006.9.

Start with the beacons.save entry point and the pillar-provided beacon path, then inspect how they produce /etc/salt/minion.d/beacons.conf. Compare this with the working beacons.add behavior. Done means a beacon defined in pillar data is written to beacons.conf and remains available after the minion restarts.