[bug]Failed to run a container with empty label

[CodeJuan]

Ⅰ. Issue Description

./bin/pouch run --label k busybox top

Error: failed to run container: invalid label k: label must be in format of key=value

Ⅱ. Describe what happened

Error: failed to run container: invalid label k: label must be in format of key=value

Ⅲ. Describe what you expected to happen

Create a label k, the value of k is ""

Ⅳ. How to reproduce it (as minimally and precisely as possible)

./bin/pouch run --label k busybox top

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):

GoVersion:       go1.12.1
KernelVersion:   4.14.0-1.el7.elrepo.x86_64
Os:              linux
Version:         1.3.0
APIVersion:      1.24
Arch:            amd64
BuildTime:       2019-04-16T02:27:23+08:00
GitCommit:       1.3.0-19-g33a9d82

• OS (e.g. from /etc/os-release):

NAME="CentOS Linux"
VERSION="7 (Core)"

• Kernel (e.g. uname -a):

4.14.0-1.el7.elrepo.x86_64

• Install tools:
• Others:

[Flsorescence]

I will try to solve this issue.