Several Pester tests for Microsoft.PowerShell.Security are incorrectly using null instead of $null

• Use of Should not be null should have used $null. Change to Should -Not -BeNullOrEmpty and remediate
  example:

  PowerShell/test/powershell/Modules/Microsoft.PowerShell.Security/CertificateProvider.Tests.ps1

  Line 51 in 76526c6

  $result | should not be null

• Use of Get-Item should be Get-ChildItem in cert provider test (Should Get-Item Cert:\ return an object #6258). Correcting $null check will expose this bug.
  

  PowerShell/test/powershell/Modules/Microsoft.PowerShell.Security/CertificateProvider.Tests.ps1

  Line 50 in 76526c6

  $result = Get-Item -LiteralPath $path

Discovered while working on #6256