Token listing `LastUsed` value is zero value string of time.Time struct

[jaythamke]

Description

Token Listing (JSON format ) LastUsed value is zero time.Time value string.

Steps to reproduce the issue:

1. Login into docker hub account via hub-tool
2. Create a token

hub-tool token create

3. List tokens in JSON format

hub-tool token ls --format json

Describe the results you received:

Describe the results you expected: LastUsed value in JSON format should be empty string if token is not used for authentication instead of zero value of time.Time{}. IMO empty string value for LastUsed in JSON would be easier way for programmatic validation for last used token value.

Additional information you deem important (e.g. issue happens only occasionally):

Happens always

Output of hub-tool --version:

Docker Hub Tool v0.3.0, build 4b740d3

Output of docker version:

Client:
 Version:           19.03.6
 API version:       1.40
 Go version:        go1.12.17
 Git commit:        369ce74a3c
 Built:             Fri Dec 18 12:21:44 2020
 OS/Arch:           linux/amd64
 Experimental:      false

Server:
 Engine:
  Version:          19.03.6
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.17
  Git commit:       369ce74a3c
  Built:            Thu Dec 10 13:23:49 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.3.3-0ubuntu1~18.04.4
  GitCommit:        
 runc:
  Version:          spec: 1.0.1-dev
  GitCommit:        
 docker-init:
  Version:          0.18.0
  GitCommit: 

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 21
  Running: 3
  Paused: 0
  Stopped: 18
 Images: 110
 Server Version: 19.03.6
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 
 runc version: 
 init version: 
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.4.0-66-generic
 Operating System: Ubuntu 18.04.5 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 1
 Total Memory: 7.776GiB
 Name: jayeshthamke-VirtualBox
 ID: 3EY5:4AIC:TAK6:VOQE:VA3E:IBW3:W56Q:EK2W:FYH7:GBCR:JYNQ:W354
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Username: cloudcijenkins
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No swap limit support

Additional environment details (AWS, VirtualBox, physical, etc.):