A JSON text must at least contain two octets

parsed = json && json.length >= 2 ? JSON.parse(json) : nil

But really the library should be able to handle this case and return nil. Web browsers with built-in JSON support seem to work just like you expect after all.

Or to do it with a only slightly intrusive mini patch:

module JSON
  def self.parse_nil(json)
    JSON.parse(json) if json && json.length >= 2
  end
end

parsed = JSON.parse_nil(json)