[CmdPal] Registry: typing "\" causes the page to throw an exception (#38151)

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## Summary of the Pull Request

Calling `string.Replace` with an empty string as the first parameter was raising an exception. Added a check to prevent this.

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Author: SeraphimaZykova

src/modules/cmdpal/Exts/Microsoft.CmdPal.Ext.Registry/Helpers/RegistryHelper.cs

@@ -53,7 +53,11 @@ internal static (IEnumerable<RegistryKey>? BaseKey, string SubKey) GetRegistryBa
         }
 
         var baseKey = query.Split('\\').FirstOrDefault() ?? string.Empty;
-        var subKey = query.Replace(baseKey, string.Empty, StringComparison.InvariantCultureIgnoreCase).TrimStart('\\');
+        var subKey = string.Empty;
+        if (!string.IsNullOrEmpty(baseKey))
+        {
+            subKey = query.Replace(baseKey, string.Empty, StringComparison.InvariantCultureIgnoreCase).TrimStart('\\');
+        }
 
         var baseKeyResult = _baseKeys
             .Where(found => found.Key.StartsWith(baseKey, StringComparison.InvariantCultureIgnoreCase))