runtime: handle empty environment variables on Plan 9

LGTM=bradfitz, rsc
R=rsc, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/137920044
runtime: don't allocate a new string in snprintf

This fixes the Plan 9 build.

Fix issue 8621.

LGTM=iant
R=rsc, mattn.jp, iant
CC=golang-codereviews
https://codereview.appspot.com/135280043