Warning with New Classic Theme Without Widgets

[SirLouen]

After some discussion in Slack I'm going to bring some context to this topic:

I never realized that a theme could have no sidebars but @audrasjb brough this:

This may be something we can improve on Core, or at least something that should be documented.

And the use-case of ArtZ91:

how I see it from my side: my goal as a developer is to create some templates based on a specific design. There are no sidebars, so I don't think I need to implement the widgets feature if it's not going to be used. Any another developer, I think, do the same for local business, and they sometimes not used widgets.

So basically, here I provide a patch to avoid the requirement of a widget sidebar area.

Testing Instructions:

1. Create a basic classic theme with only style.css and index.php, can follow these two files provided in docs instructions: https://developer.wordpress.org/themes/classic-themes/your-first-theme/#classic-theme
2. Activate such theme
3. Go to the theme Customizer
4. Check server debug.log
5. Before patch, a warning should appear. After patch warning should be sorted.

Trac ticket: https://core.trac.wordpress.org/ticket/63151

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[mukeshpanchal27]

Add messages for multiple asserts so if it get failed it return meaningful message.