Fix a viewer error that occurs when AWS is not used.

[sutatoruta]

Even if AWS credentials config values are nil, $config.has_key? :aws returns true.
It might not be working as expected:

slack-patron/viewer/viewer.rb

Lines 13 to 22 in c86c477

	if $config.has_key? :aws
	Aws.config.update({
	credentials: Aws::Credentials.new(
	$config[:aws][:access_key_id],
	$config[:aws][:secret_access_key],
	),
	region: 'ap-northeast-1',
	})
	$signer = Aws::S3::Presigner.new
	end

Since $singer is not nil, presigned_url is called without AWS credentials set and throws Aws::Sigv4::Errors::MissingCredentialsError:

slack-patron/viewer/viewer.rb

Lines 29 to 56 in c86c477

	def sign_file(file)
	if file.has_key? 'url_private_download'
	url = $signer.presigned_url(:get_object, {
	bucket: 'tsgbot-slack-files',
	key: file['id'],
	expires_in: 180,
	})
	file['url_private_download'] = url
	end
	end
	def sign_message_files(messages)
	unless $signer.nil?
	messages.each do |message|
	if message.has_key? 'files'
	message['files'].each do |file|
	sign_file(file)
	end
	end
	# Compatibility with old messages
	if message.has_key? 'file'
	file = message['file']
	sign_file(file)
	end
	end
	end
	end

I fixed this problem by correcting $config.has_key? :aws to $config[:aws]&.values.all?.

[tyage]

thank you!
looks fine!