Make sure we always return an object in command searcher (#9623)

src/System.Management.Automation/engine/CommandSearcher.cs

@@ -565,7 +565,7 @@ namespace System.Management.Automation
             }
 
             provider = null;
-            return null;
+            return new Collection<string>();
         }
 
         private static bool checkPath(string path, string commandName)

test/powershell/engine/Basic/CommandDiscovery.Tests.ps1

@@ -205,4 +205,10 @@ Describe "Command Discovery tests" -Tags "CI" {
             }
         }
     }
+
+    Context "error cases" {
+        It 'Get-Command "less `"-PsPage %db?B of %DoesNotExist:`"" should throw Drive not found' {
+            {Get-Command -Name "less `"-PsPage %db?B of %DoesNotExist:`""} | Should -Throw -ErrorId 'DriveNotFound' -Because "The drive 'DoesNotExist:' should not exist"
+        }
+    }
 }