juju does not handle empty configuration variables

[Matt Bruzek]

I was performing a test on the current version of Juju (1.19.4) and I found an error with setting a configuration variable to the empty string.

I ran a script that attempted to set a configuration variable to the empty string
juju set cassandra apt-repo-spec=""

And got the following error.
ubuntu@stilson-01:~$ mbruzek/deploy_cassandra.sh
+ juju deploy --repository=/home/ubuntu/charms local:trusty/cassandra
Added charm "local:trusty/cassandra-5" to the environment.
+ juju set cassandra apt-repo-spec=
panic: runtime error: index out of range

goroutine 1 [running]:
main.Run.pN15_main.SetCommand
        /build/buildd/juju-core-1.19.4/src/github.com/juju/juju/cmd/juju/set.go:92
github.com_juju_juju_cmd_envcmd.Run.N53_github.com_juju_juju_cmd_envcmd.environCommandWrapper
        /build/buildd/juju-core-1.19.4/src/github.com/juju/juju/cmd/envcmd/environmentcommand.go:102
main.Run.N18_main.envCmdWrapper
        /build/buildd/juju-core-1.19.4/src/github.com/juju/juju/cmd/juju/main.go:153
github.com_juju_cmd.Run.pN32_github.com_juju_cmd.SuperCommand
        /build/buildd/juju-core-1.19.4/src/github.com/juju/cmd/supercommand.go:321
github.com_juju_cmd.Main
        /build/buildd/juju-core-1.19.4/src/github.com/juju/cmd/cmd.go:247
main.Main
        /build/buildd/juju-core-1.19.4/src/github.com/juju/juju/cmd/juju/main.go:77
main.main
        /build/buildd/juju-core-1.19.4/src/github.com/juju/juju/cmd/juju/main.go:173

goroutine 3 [syscall]:
        goroutine in C code; stack unavailable
+ juju add-unit -n 2 cassandra
ubuntu@stilson-01:~$ juju --version
1.19.4-trusty-ppc64

I can reproduce this problem by setting other variables to empty string.