Don't use tmpdir directory if environment variable is not defined

Right now we fallback to a temporary directory in https://gitlab.com/gitlab-org/prometheus-client-mmap/blob/ec86af4e8d09eac7192742a701d6a7f2fe7e1d8c/lib%2Fprometheus%2Fclient%2Fconfiguration.rb#L17 if prometheus_multiproc_dir is not defined.

We should probably not do this to avoid issues such as gitlab-org/gitlab#281033 (comment 461190648). Probably better just to write to /dev/null?