fix core list --all sometimes crashing

[umbynos]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

bug fix

• What is the current behavior?

I discovered that sometimes some residue can be left behind, after a core uninstall, for example:

tree ~/.arduino15/packages/
/home/umberto/.arduino15/packages/
├── attiny
│   └── hardware
│       └── avr

The package manager tries to load it as a package and this results in a dirty environment, which causes the cli to crash when running core list --all
further details in #1434

• What is the new behavior?

a package with empty tools and platform is now not considered by the package manager
closes #1434

• Does this PR introduce a breaking change, and is
  titled accordingly?

nope

• Other information:

---

See how to contribute

[umbynos]

Apparently the problem is still present when using indexes not perfectly compliant with specs (e.g. https://raw.githubusercontent.com/keyboardio/ArduinoCore-GD32-Keyboardio/main/package_gd32_index.json)
In this index the checksum and the size are missing (see #1468). Also the signature for that index is missing but this is another problem.
Apparently adding this index with config add causes the parsing when calling core list --all to fail because the platform does not has a release

[per1234]

Thanks Umberto!