vault audit list -format json does not return JSON when list is empty

[netson]

Describe the bug
The command vault audit list -format json does not consistently return a valid JSON object; if there are no audits enabled, the returned result is simply a string. When trying to automate certain steps, always receiving a JSON object is mandatory.

To Reproduce
Steps to reproduce the behavior:

1. make sure no audits are enabled (vault audit disable ***)
2. run vault audit list -format json
3. you will see the following:
   No audit devices are enabled.

Expected behavior
I would expect to see an empty JSON object: {}

Environment:

• Vault Server Version (retrieve with vault status):

# vault status
Key             Value
---             -----
Seal Type       shamir
Initialized     true
Sealed          false
Total Shares    5
Threshold       3
Version         1.1.2
Cluster Name    vault-cluster-3e8e9591
Cluster ID      7110de04-06b4-e73d-c2ce-94ac9616cde6
HA Enabled      false

• Vault CLI Version (retrieve with vault version):

Vault v1.1.2 ('0082501623c0b704b87b1fbc84c2d725994bac54')

• Server Operating System/Architecture:

# lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 18.04.2 LTS
Release:	18.04
Codename:	bionic

• Vault server configuration file(s):

# cat /etc/vault.d/vault.hcl
api_addr = "http://127.0.0.1:8201"
ui = true

backend "file" {
    path = "/opt/vault"
}

listener "tcp" {
    address = "127.0.0.1:8200"
    tls_disable = 1
}

Additional context
None