go.tools/dashboard: don't do method calls or field lookups on a possibly nil repo
go.tools/dashboard: don't do method calls or field lookups on a possibly nil repo

LGTM=mischief, adg
R=adg, mischief
CC=golang-codereviews
https://golang.org/cl/106660043

proppy/build,evandbrown/build,golang/build,golang/build,evandbrown/build,proppy/build,proppy/build,evandbrown/build,golang/build,golang/build,proppy/build