juju deploy panics with invalid --to option

Not a big issue, but maybe worth some input sanitizing:

root@server:~# juju --version
2.9.7-ubuntu-amd64
root@server:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 20.04.2 LTS
Release: 20.04
Codename: focal

Accidentally used "--to 0,1,2," with the trailing comma, which caused a panic.

root@server:~# juju deploy -n 3 --to 0,1,2, --config vip=10.0.0.101 keystone
WARNING 3 unit(s) will be deployed, extra placement directives will be ignored
Located charm "keystone" in charm-hub, revision 533
Deploying "keystone" from charm-hub charm "keystone", revision 533 in channel stable
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1c54ce6]

goroutine 1 [running]:
github.com/juju/juju/cmd/juju/application.(*deployAPIAdapter).Deploy(0xc0003b47e0, 0xc000914840, 0xc00022dff0, 0x9, 0xc00022dff9, 0x5, 0xc0003757a0, 0x20, 0xc000081840, 0x40, ...)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/application/deploy.go:129 +0x66
github.com/juju/juju/cmd/juju/application/deployer.(*deployCharm).deploy(0xc000700380, 0xc00050df90, 0x7efd8aa4f2e0, 0xc0003b47e0, 0x0, 0x0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/application/deployer/charm.go:255 +0x1086
github.com/juju/juju/cmd/juju/application/deployer.(*repositoryCharm).PrepareAndDeploy(0xc000700380, 0xc00050df90, 0x7efd8aa4f2e0, 0xc0003b47e0, 0x4e25f40, 0xc000295aa0, 0x4d9e000, 0xc000293950, 0x0, 0x0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/application/deployer/charm.go:536 +0xd9b
github.com/juju/juju/cmd/juju/application.(*DeployCommand).Run(0xc00031f500, 0xc00050df90, 0x0, 0x0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/application/deploy.go:815 +0x776
github.com/juju/juju/cmd/modelcmd.(*modelCommandWrapper).Run(0xc00028e690, 0xc00050df90, 0x7059020, 0x3bd4d60)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/modelcmd/modelcommand.go:651 +0x10a
github.com/juju/juju/cmd/modelcmd.(*baseCommandWrapper).Run(0xc0002b03e0, 0xc00050df90, 0x0, 0x0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/modelcmd/base.go:546 +0xa8
github.com/juju/cmd.(*SuperCommand).Run(0xc0004a9900, 0xc00050df90, 0xc00050df90, 0x0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/vendor/github.com/juju/cmd/supercommand.go:523 +0x2ba
github.com/juju/cmd.Main(0x4e720c0, 0xc0004a9900, 0xc00050df90, 0xc000167a00, 0x8, 0x8, 0x0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/vendor/github.com/juju/cmd/cmd.go:387 +0x2bf
github.com/juju/juju/cmd/juju/commands.jujuMain.Run(0x46f7ee0, 0xc000138000, 0x9, 0x9, 0xc00000dda0)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/commands/main.go:200 +0x424
github.com/juju/juju/cmd/juju/commands.Main(...)
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/commands/main.go:123
main.main()
        /build/snapcraft-juju-35d6cf/parts/juju/src/cmd/juju/main.go:37 +0xb9

Command ran fine after fixing the --to option:
root@server:~# juju deploy -n 3 --to 0,1,2 --config vip=10.0.0.101 keystone
Located charm "keystone" in charm-hub, revision 533
Deploying "keystone" from charm-hub charm "keystone", revision 533 in channel stable