Fix HTTParty deprecation warning

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HTTParty is printing a deprecation warning anytime we use the LLM client:

[DEPRECATION] HTTParty will no longer override `response#nil?`. This functionality will be removed in future versions. Please, add explicit check `response.body.nil? || response.body.empty?`. For more info refer to: https://github.com/jnunemaker/httparty/issues/568

This change simply replaces the deprecated call with the recommended replacement.

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