Fix issue when completing the first command in a script with an empty array expression (#18355) (#18630)

Co-authored-by: MartinGC94 <42123497+MartinGC94@users.noreply.github.com>

src/System.Management.Automation/engine/CommandCompletion/CompletionAnalysis.cs

@@ -1986,7 +1986,10 @@ namespace System.Management.Automation
                 // Handle completion for a path with variable, such as: $PSHOME\ty<tab>
                 if (completionContext.RelatedAsts.Count > 0 && completionContext.RelatedAsts[0] is ScriptBlockAst)
                 {
-                    Ast cursorAst = completionContext.RelatedAsts[0].FindAll(ast => ast.Extent.EndOffset <= tokenAtCursor.Extent.StartOffset, true).LastOrDefault();
+                    Ast cursorAst = completionContext.RelatedAsts[0].FindAll(
+                        ast => ast.Extent.EndOffset <= tokenAtCursor.Extent.StartOffset
+                            && ast.Extent is not EmptyScriptExtent,
+                        searchNestedScriptBlocks: true).LastOrDefault();
 
                     if (cursorAst is not null)
                     {

test/powershell/Host/TabCompletion/TabCompletion.Tests.ps1

@@ -759,6 +759,11 @@ ConstructorTestClass(int i, bool b)
         $res.CompletionMatches.CompletionText | Should -Contain "-Path"
     }
 
+    it 'Should complete command with an empty arrayexpression element' {
+        $res = TabExpansion2 -inputScript 'Get-ChildItem @()' -cursorColumn 1
+        $res.CompletionMatches[0].CompletionText | Should -Be "Get-ChildItem"
+    }
+
     Context "Script name completion" {
         BeforeAll {
             Setup -f 'install-powershell.ps1' -Content ""