[Hexpm] Fix badges for pre-release only versions

[sheerlox]

Hex.pm's API now seems to return null for latest_stable_version, where I assume from shields' code that it previously did not return the property at all:

  },
  "html_url": "https://hex.pm/packages/nodelix",
  "latest_stable_version": null
}

source

This PR aims to support that in order for badges of pre-release-only version packages work again.

Example of a broken badge:

Error logs from npm run badge -- /hexpm/dt/nodelix on master:

 Response did not match schema  🤷 
 "latest_stable_version" must be a string
 Handled error  🙅 
 [ValidationError: "latest_stable_version" must be a string] {
  prettyMessage: 'invalid response data',
  cacheSeconds: undefined,
  response: undefined
}

[github-actions]

	Messages
📖	✨ Thanks for your contribution to Shields, @sheerlox!

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[github-actions]

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[chris48s]

thanks 👍