Guillaume Tardif 819a7389f7 Fix login error when login to an azure account having no associated tenant.
This was encountered by @lorenrh with stack : 
```
docker login azure
panic: runtime error: index out of range [0] with length 0
goroutine 1 [running]:
github.com/docker/api/azure/login.AzureLoginService.Login(0xc0004cc2a0, 0x2d, 0xe8b8a0, 0x1476830, 0xe8e4e0, 0xc000498ff0, 0x0, 0x0)
        github.com/docker/api/azure/login/login.go:130 +0x1057
github.com/docker/api/azure.(*aciCloudService).Login(0xc0004888c0, 0xe8e4e0, 0xc000498ff0, 0x0, 0xe82720, 0xc0004888c0)
        github.com/docker/api/azure/backend.go:283 +0x64
github.com/docker/api/cli/cmd/login.cloudLogin(0xc0004eb600, 0xd422a4, 0x3, 0x1, 0xffffffffffffffff)
        github.com/docker/api/cli/cmd/login/login.go:53 +0xae
github.com/docker/api/cli/cmd/login.runLogin(0xc0004eb600, 0xc00049c5f0, 0x1, 0x1, 0x0, 0x0)
        github.com/docker/api/cli/cmd/login/login.go:39 +0x15e
github.com/spf13/cobra.(*Command).execute(0xc0004eb600, 0xc00049c5d0, 0x1, 0x1, 0xc0004eb600, 0xc00049c5d0)
        github.com/spf13/cobra@v1.0.0/command.go:842 +0x45a
github.com/spf13/cobra.(*Command).ExecuteC(0xc0004a8580, 0xc000498fc0, 0xc41160, 0x1476830)
        github.com/spf13/cobra@v1.0.0/command.go:950 +0x350
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/cobra@v1.0.0/command.go:887
github.com/spf13/cobra.(*Command).ExecuteContext(...)
        github.com/spf13/cobra@v1.0.0/command.go:880
main.main()
        github.com/docker/api/cli/main.go:167 +0x68f
```
2020-06-10 10:15:03 +02:00
..
2020-06-04 10:20:39 +02:00