Address Copilot review: fix nullability and add whitespace tests

framework/src/Volo.Abp.AspNetCore.MultiTenancy/Volo/Abp/AspNetCore/MultiTenancy/QueryStringTenantResolveContributor.cs

@@ -21,7 +21,7 @@ public class QueryStringTenantResolveContributor : HttpTenantResolveContributorB
                 var tenantValue = httpContext.Request.Query[tenantKey].ToString();
                 if (!tenantValue.IsNullOrWhiteSpace())
                 {
-                    return Task.FromResult(tenantValue)!;
+                    return Task.FromResult<string?>(tenantValue);
                 }
             }
         }

framework/test/Volo.Abp.AspNetCore.MultiTenancy.Tests/Volo/Abp/AspNetCore/MultiTenancy/AspNetCoreMultiTenancy_Without_DomainResolver_Tests.cs

@@ -86,4 +86,20 @@ public class AspNetCoreMultiTenancy_Without_DomainResolver_Tests : AspNetCoreMul
         var result = await GetResponseAsObjectAsync<Dictionary<string, string>>($"http://abp.io?{_options.TenantKey}=");
         result["TenantId"].ShouldBe("");
     }
+
+    [Fact]
+    public async Task Should_Use_Header_Tenant_Id_When_QueryString_Tenant_Is_Whitespace()
+    {
+        Client.DefaultRequestHeaders.Add(_options.TenantKey, _testTenantId.ToString());
+
+        var result = await GetResponseAsObjectAsync<Dictionary<string, string>>($"http://abp.io?{_options.TenantKey}=%20");
+        result["TenantId"].ShouldBe(_testTenantId.ToString());
+    }
+
+    [Fact]
+    public async Task Should_Fallback_To_Host_When_QueryString_Tenant_Is_Whitespace_And_No_Other_Resolver()
+    {
+        var result = await GetResponseAsObjectAsync<Dictionary<string, string>>($"http://abp.io?{_options.TenantKey}=%20");
+        result["TenantId"].ShouldBe("");
+    }
 }