Pop up an error when deploying images to Azure App Service for the private registry with no authentication

[v-xuanzh]

Version: 1.29.0-insider (user setup)
Node.js: 8.9.3
Architecture: x64
Extension:

Repro Steps:

1. Start up a private local registry with the command:
   
2. Right click Private Registry -> Connect to a Private Registry...(Preview)
3. URL: http://localhost:5100
   Username: Press Enter
4. Select an existing image to push the current private registry.
5. Right click the pushed image -> 'Deploy Image to Azure App Service'.
6. Select valid resource group, app service plan and unique name for the web app.
7. Check the result.

Expect:
There is no error.

Actual:
Pop up the error with 'Cannot read property 'adminUserEnabled' of undefined'.