(GH-11175) Update terminology to 'enumerable null'

[michaeltlombardi]

PR Summary

Prior to this change, the article Everything you wanted to know about $null used the term 'empty $null' to describe [System.Management.Automation.Internal.AutomationNull]::Value

However, this terminology isn't clear enough, as both that value and the $null value itself can be considered to be 'empty.'

This change:

• Updates to use the terminology 'enumerable null' instead of 'empty $null' to describe the value.
• Resolves #11175
• Fixes AB#260888

PR Checklist

• [x] Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• [x] Summary: This PR's summary describes the scope and intent of the change.
• [x] Contributor's Guide: I have read the contributors guide.
• [x] Style: This PR adheres to the style guide.

[learn-build-service-prod[bot]]

Learn Build status updates of commit ef92c56:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/learn/deep-dives/everything-about-null.md	:white_check_mark:Succeeded	View (>=powershell-5.1)

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[mklement0]

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