/opt/bitnami/apps/wordpress/htdocs/wp-config.php file does not seem to be empty. You should see its content by running:
sudo cat /opt/bitnami/apps/wordpress/htdocs/wp-config.php
You also can add a line with this:
sudo echo -e "\nMY_NEW_LINE" >> /opt/bitnami/apps/wordpress/htdocs/wp-config.php
Substitute MY_NEW_LINE with the content you would like to add.
I hope it helps