Omit empty Template in configuration

[lawliet89]

When using agent cache only without any secrets defined via template annotations, the configuration generated has null for the template key, leading to Vault agent crashing.

For example:

{
  "auto_auth": {
    "method": {
      "type": "kubernetes",
      "mount_path": "auth/kubernetes",
      "config": {
        "role": "app"
      }
    },
    "sink": [
      {
        "type": "file",
        "config": {
          "path": "/home/vault/.vault-token"
        }
      }
    ]
  },
  "exit_after_auth": false,
  "pid_file": "/home/vault/.pid",
  "vault": {
    "address": "https://vault.default.svc:8200",
    "ca_cert": "/vault/tls/ca.crt",
    "tls_server_name": "vault.default.svc.cluster.local"
  },
  "template": null,
  "listener": [
    {
      "type": "tcp",
      "address": "127.0.0.1:8200",
      "tls_disable": true
    }
  ],
  "cache": {
    "use_auto_auth_token": "true"
  }
}

This PR simply omits the template key if it's empty.

[jasonodonnell]

LGTM, thanks!