Tor systemd service should have RuntimeDirectory=tor
Tor writes it's pidfile to /var/run/tor/tor.pid by default. Therefore, the systemd configuration should create this directory with appropriate permissions by default.
In https://gitweb.torproject.org/tor.git/tree/contrib/dist/tor.service.in there should be this line: {{{ }}} added to the [Service] section.
See the documentation at https://www.freedesktop.org/software/systemd/man/systemd.exec.html for more detail.
Trac:
Username: candrews