Handle cases where the there are no changed files (#26500)

In PS using "| Where-Object" on a null acts like
there is one null object in the list so trying
access properties on it fails. Instead use the
"Where()" operator so it will treat null as an
empty list as it should.
 This is a combination of 4 commits.

Update ChangedFiles-Functions.ps1

Update ChangedFiles-Functions.ps1

Update ChangedFiles-Functions.ps1

Update ChangedFiles-Functions.ps1

Author: weshaggard

eng/scripts/ChangedFiles-Functions.ps1

@@ -10,6 +10,7 @@ function Get-ChangedFiles($baseCommitish = "HEAD^", $targetCommitish = "HEAD", $
   # For PR's that last commit is always a merge commit so HEAD^ will get the parent
   # commit of the base branch and as such will diff HEAD against HEAD^
   $changedFiles = git -c core.quotepath=off diff --name-only --diff-filter=$diffFilter $baseCommitish $targetCommitish
+  $changedFiles = $changedFiles | Where-Object { !$_.Contains("ChangedFiles-Functions") }
 
   Write-Verbose "Changed files:"
   $changedFiles | ForEach-Object { Write-Verbose "$_" }
@@ -20,17 +21,17 @@ function Get-ChangedFiles($baseCommitish = "HEAD^", $targetCommitish = "HEAD", $
 function Get-ChangedSwaggerFiles() {
   $changedFiles = Get-ChangedFilesUnderSpecification
 
-  $changedSwaggerFiles = $changedFiles | Where-Object { 
+  $changedSwaggerFiles = $changedFiles.Where({ 
     $_.EndsWith(".json")
-  }
+  })
 
   return $changedSwaggerFiles
 }
 
 function Get-ChangedFilesUnderSpecification($changedFiles = (Get-ChangedFiles)) {
-  $changedFilesUnderSpecification = $changedFiles | Where-Object { 
+  $changedFilesUnderSpecification = $changedFiles.Where({ 
     $_.StartsWith("specification")
-  }
+  })
 
   return $changedFilesUnderSpecification
 }
@@ -44,11 +45,11 @@ function Get-ChangedCoreFiles($changedFiles = (Get-ChangedFiles)) {
     "tsconfig.json"
   )
 
-  $coreFiles = $changedFiles | Where-Object { 
+  $coreFiles = $changedFiles.Where({ 
     $_.StartsWith("eng/") -or
     $_.StartsWith("specification/common-types/") -or
     $_ -in $rootFiles
-  }
+  })
 
   return $coreFiles
 }