Handle null in Region/ServiceURL setters and tighten nullable hints

- Clear configuration entry when Region or ServiceURL is set to null
- Annotate DeleteObjectsAsync prefix and continuationToken as nullable
- Replace vague Region placeholder in aws.md example

docs/en/framework/infrastructure/blob-storing/aws.md

@@ -41,7 +41,7 @@ Configure<AbpBlobStoringOptions>(options =>
             Aws.UseTemporaryFederatedCredentials = "set true to use temporary federated credentials";
             Aws.ProfileName = "the name of the profile to get credentials from";
             Aws.ProfilesLocation = "the path to the aws credentials file to look at";
-            Aws.Region = "the system name of the service";
+            Aws.Region = "the AWS region system name, e.g. us-east-1";
             Aws.ServiceURL = "custom service URL for S3-compatible APIs (optional)";
             Aws.Name = "the name of the federated user";
             Aws.Policy = "policy";

framework/src/Volo.Abp.BlobStoring.Aws/Volo/Abp/BlobStoring/Aws/AwsBlobProviderConfiguration.cs

@@ -64,7 +64,15 @@ public class AwsBlobProviderConfiguration
     /// </summary>
     public string? Region {
         get => _containerConfiguration.GetConfigurationOrDefault<string>(AwsBlobProviderConfigurationNames.Region);
-        set => _containerConfiguration.SetConfiguration(AwsBlobProviderConfigurationNames.Region, value);
+        set
+        {
+            if (value == null)
+            {
+                _containerConfiguration.ClearConfiguration(AwsBlobProviderConfigurationNames.Region);
+                return;
+            }
+            _containerConfiguration.SetConfiguration(AwsBlobProviderConfigurationNames.Region, value);
+        }
     }
 
     /// <summary>
@@ -74,7 +82,15 @@ public class AwsBlobProviderConfiguration
     /// </summary>
     public string? ServiceURL {
         get => _containerConfiguration.GetConfigurationOrDefault<string>(AwsBlobProviderConfigurationNames.ServiceURL);
-        set => _containerConfiguration.SetConfiguration(AwsBlobProviderConfigurationNames.ServiceURL, value);
+        set
+        {
+            if (value == null)
+            {
+                _containerConfiguration.ClearConfiguration(AwsBlobProviderConfigurationNames.ServiceURL);
+                return;
+            }
+            _containerConfiguration.SetConfiguration(AwsBlobProviderConfigurationNames.ServiceURL, value);
+        }
     }
 
     /// <summary>

framework/test/Volo.Abp.BlobStoring.Aws.Tests/Volo/Abp/BlobStoring/Aws/AbpBlobStoringAwsTestModule.cs

@@ -139,9 +139,9 @@ public class AbpBlobStoringAwsTestModule : AbpModule
         }
     }
 
-    private async Task DeleteObjectsAsync(AmazonS3Client client, string prefix)
+    private async Task DeleteObjectsAsync(AmazonS3Client client, string? prefix)
     {
-        string continuationToken = null;
+        string? continuationToken = null;
         do
         {
             var listResponse = await client.ListObjectsV2Async(new ListObjectsV2Request