Target parsing panics when name is not set

[lgfa29]

If name is not set in the target block of a policy the agent will panic.

Log output

2020-04-01T13:16:02.456-0400 [INFO]  agent: reading policies: policy_storage=policystorage.Nomad
2020-04-01T13:16:02.458-0400 [INFO]  agent: found 1 policies: policy_storage=policystorage.Nomad
panic: interface conversion: interface {} is nil, not string

goroutine 131 [running]:
github.com/hashicorp/nomad-autoscaler/policystorage.parseTarget(0x17000a0, 0xc0004626c0, 0x183ae3d)
        /Users/laoqui/go/src/github.com/hashicorp/nomad-autoscaler/policystorage/nomad.go:143 +0x3d8
github.com/hashicorp/nomad-autoscaler/policystorage.(*Nomad).Get(0xc00000e028, 0xc000375da0, 0x24, 0x2, 0x1, 0x13c4d00)
        /Users/laoqui/go/src/github.com/hashicorp/nomad-autoscaler/policystorage/nomad.go:53 +0x266
github.com/hashicorp/nomad-autoscaler/agent.(*Agent).Run.func1(0xc00014c040, 0x1984400, 0xc0001508c0, 0xc00000e028, 0x198ff60, 0xc0003c6a50, 0xc00021a2d0, 0xc000375da0, 0x24)
        /Users/laoqui/go/src/github.com/hashicorp/nomad-autoscaler/agent/agent.go:107 +0xc3
created by github.com/hashicorp/nomad-autoscaler/agent.(*Agent).Run
        /Users/laoqui/go/src/github.com/hashicorp/nomad-autoscaler/agent/agent.go:101 +0x60f

Sample policy

policy {
  source = "prometheus"
  query  = "scalar(avg((haproxy_server_current_sessions{backend=\"http_back\"}) and (haproxy_server_up{backend=\"http_back\"} == 1)))"

  strategy = {
    name = "target-value"

    config = {
      target = 20
    }
  }

  target {
    config {
      dry-run = "true"
    }
  }
}

Expected behaviour:

Target defaults to local Nomad cluster.