The /etc/apache2/httpd.conf is empty in Ubuntu, because the Apache configuration resides in /etc/apache2/apache2.conf!
“httpd.conf is for user options.” No it isn’t, it’s there for historic reasons.
Using Apache server, all user options should go into a new *.conf-file inside /etc/apache2/conf.d/. This method should be “update-safe”, as httpd.conf or apache2.conf may get overwritten on the next server update.
Inside /etc/apache2/apache2.conf, you will find the following line, which includes those files:
# Include generic snippets of statements
Include conf.d/
As of Apache 2.4+ the user configuration directory is /etc/apache2/conf-available/. Use a2enconf FILENAME_WITHOUT_SUFFIX to enable the new configuration file or manually create a symlink in /etc/apache2/conf-enabled/. Be aware that as of Apache 2.4 the configuration files must have the suffix .conf (e.g. conf-available/my-settings.conf);