Clarify null value output in PowerShell

Wrapped output in single quotes because we don't allow trailing spaces in our documents.

Author: sdwheeler

reference/docs-conceptual/learn/deep-dives/everything-about-null.md

@@ -61,8 +61,8 @@ If you use `$null` in a string, then it's a blank value (or empty string).
 
 ```powershell
 PS> $value = $null
-PS> Write-Output "The value is $value"
-The value is 
+PS> Write-Output "'The value is $value'"
+'The value is '
 ```
 
 This is one of the reasons that I like to place brackets around variables when using them in log