Use GetOrNullAsync for feature definition lookup

Replaces GetAsync with GetOrNullAsync when retrieving feature definitions in FeatureChecker. This prevents exceptions when a feature is not found and returns null instead, improving error handling.

framework/src/Volo.Abp.Features/Volo/Abp/Features/FeatureChecker.cs

@@ -28,7 +28,12 @@ public class FeatureChecker : FeatureCheckerBase
 
     public override async Task<string?> GetOrNullAsync(string name)
     {
-        var featureDefinition = await FeatureDefinitionManager.GetAsync(name);
+        var featureDefinition = await FeatureDefinitionManager.GetOrNullAsync(name);
+        if (featureDefinition == null)
+        {
+            return null;
+        }
+        
         var providers = FeatureValueProviderManager.ValueProviders
             .Reverse();