Fix alertmanager.cluster.listen-address with single replica

[hamishforbes]

Looks like this got broken again in some refactoring along the way
Setting the flag to an empty string causes an error on startup:

level=error ts=2021-11-29T02:53:40.522651402Z caller=log.go:106 msg="error running cortex" err="address \"\": missing port in address
invalid listen address
github.com/prometheus/alertmanager/cluster.Create
	/__w/cortex/cortex/vendor/github.com/prometheus/alertmanager/cluster/cluster.go:147
github.com/cortexproject/cortex/pkg/alertmanager.NewMultitenantAlertmanager
	/__w/cortex/cortex/pkg/alertmanager/multitenant.go:317
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initAlertManager
	/__w/cortex/cortex/pkg/cortex/modules.go:723
github.com/grafana/dskit/modules.(*Manager).initModule
	/__w/cortex/cortex/vendor/github.com/grafana/dskit/modules/modules.go:106
github.com/grafana/dskit/modules.(*Manager).InitModuleServices
	/__w/cortex/cortex/vendor/github.com/grafana/dskit/modules/modules.go:78
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).Run
	/__w/cortex/cortex/pkg/cortex/cortex.go:412
main.main
	/__w/cortex/cortex/cmd/cortex/main.go:195
runtime.main
	/usr/local/go/src/runtime/proc.go:225
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1371
unable to initialize gossip mesh
github.com/cortexproject/cortex/pkg/alertmanager.NewMultitenantAlertmanager
	/__w/cortex/cortex/pkg/alertmanager/multitenant.go:332
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initAlertManager
	/__w/cortex/cortex/pkg/cortex/modules.go:723
github.com/grafana/dskit/modules.(*Manager).initModule
	/__w/cortex/cortex/vendor/github.com/grafana/dskit/modules/modules.go:106
github.com/grafana/dskit/modules.(*Manager).InitModuleServices
	/__w/cortex/cortex/vendor/github.com/grafana/dskit/modules/modules.go:78
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).Run
	/__w/cortex/cortex/pkg/cortex/cortex.go:412
main.main
	/__w/cortex/cortex/cmd/cortex/main.go:195
runtime.main
	/usr/local/go/src/runtime/proc.go:225
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1371
error initialising module: alertmanager
github.com/grafana/dskit/modules.(*Manager).initModule
	/__w/cortex/cortex/vendor/github.com/grafana/dskit/modules/modules.go:108
github.com/grafana/dskit/modules.(*Manager).InitModuleServices
	/__w/cortex/cortex/vendor/github.com/grafana/dskit/modules/modules.go:78
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).Run
	/__w/cortex/cortex/pkg/cortex/cortex.go:412
main.main
	/__w/cortex/cortex/cmd/cortex/main.go:195
runtime.main
	/usr/local/go/src/runtime/proc.go:225
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1371"

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