Replaced duplicate link and improved examples for $null

[sdwheeler]

• Fixes AB#304272 - replace duplicate link
• Fixes AB#304282 - improve wording and example

[learn-build-service-prod]

Learn Build status updates of commit c4bcc02:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/PSScriptAnalyzer/rules-recommendations.md	✅Succeeded	View (ps-modules)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 0a6ec4c:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/docs-conceptual/PSScriptAnalyzer/rules-recommendations.md	✅Succeeded	View (ps-modules)
reference/docs-conceptual/PSScriptAnalyzer/Rules/PossibleIncorrectComparisonWithNull.md	✅Succeeded	View (ps-modules)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel