[bug] pouch pull can't handle space input

[Letty5411]

Ⅰ. Issue Description

The following CMD will panic: pouch pull " "

Ⅱ. Describe what happened

#pouch pull " "
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x7944f2]

goroutine 1 [running]:
github.com/alibaba/pouch/pkg/reference.(*taggedReference).Name(0xc420223220, 0xc3dd20, 0xc420223220)
  <autogenerated>:1 +0x32
main.pullMissingImage(0xc40160, 0xc420014098, 0xc45c00, 0xc42009f1d0, 0x7ffd41c9a5ec, 0x1, 0xc42009f101, 0x0, 0x0)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/cli/pull.go:212 +0x146
main.(*PullCommand).runPull(0xc420427d80, 0xc4203eaaa0, 0x1, 0x1, 0x0, 0x0)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/cli/pull.go:60 +0x7d
main.(*PullCommand).Init.func1(0xc420216d80, 0xc4203eaaa0, 0x1, 0x1, 0x0, 0x0)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/cli/pull.go:46 +0x48
github.com/alibaba/pouch/vendor/github.com/spf13/cobra.(*Command).execute(0xc420216d80, 0xc4203eaa60, 0x1, 0x1, 0xc420216d80, 0xc4203eaa60)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/vendor/github.com/spf13/cobra/command.go:742 +0x49b
github.com/alibaba/pouch/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc420216b40, 0xc420293e98, 0x1, 0x1)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/vendor/github.com/spf13/cobra/command.go:827 +0x30e
github.com/alibaba/pouch/vendor/github.com/spf13/cobra.(*Command).Execute(0xc420216b40, 0x1, 0xc4203ea9f0)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/vendor/github.com/spf13/cobra/command.go:780 +0x2b
main.(*Cli).Run(0xc420595400, 0xc3ce20, 0xc420427d70)
  /tmp/pouchbuild/src/github.com/alibaba/pouch/cli/cli.go:95 +0x2f
main.main()
  /tmp/pouchbuild/src/github.com/alibaba/pouch/cli/main.go:53 +0xa9f

Ⅲ. Describe what you expected to happen

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

Ⅴ. Anything else we need to know?

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others: