Restore Client-Side libraries doesn't fail when required field libraries is not specified

[bhsubra]

Functional impact

Restore Client-Side libraries doesn't fail when required field- libraries is not specified

Minimal repro steps

1. Create libman.json file with below contents:

{
  "version": "1.0",
  "defaultProvider": "filesystem"
}

2. Restore client side libraries

Expected result

We should validate and write error to output saying - required file wasn't specified or something along those lines.

Actual result

Output window has below contents:
Restore operation started...
Restoring libraries for project WebApplication5
Restore operation completed
Restore completed. Files already up-to-date

Time Elapsed: 00:00:00.0474544
========== Finished ==========