Avoid Task<string> being null.

https://github.com/abpframework/abp/pull/5849/files

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

@@ -12,7 +12,7 @@ namespace Volo.Abp.AspNetCore.MultiTenancy
 
         protected override Task<string> GetTenantIdOrNameFromHttpContextOrNullAsync(ITenantResolveContext context, HttpContext httpContext)
         {
-            return Task.FromResult(httpContext.Request?.Cookies[context.GetAbpAspNetCoreMultiTenancyOptions().TenantKey]);
+            return Task.FromResult(httpContext.Request.Cookies[context.GetAbpAspNetCoreMultiTenancyOptions().TenantKey]);
         }
     }
 }

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

@@ -30,7 +30,7 @@ namespace Volo.Abp.AspNetCore.MultiTenancy
 
             context.Handled = true;
 
-            return extractResult.IsMatch ? Task.FromResult(extractResult.Matches[0].Value) : null;
+            return Task.FromResult(extractResult.IsMatch ? extractResult.Matches[0].Value : null);
         }
     }
 }

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

@@ -18,7 +18,7 @@ namespace Volo.Abp.AspNetCore.MultiTenancy
         {
             if (httpContext.Request.Headers.IsNullOrEmpty())
             {
-                return null;
+                return Task.FromResult((string)null);
             }
 
             var tenantIdKey = context.GetAbpAspNetCoreMultiTenancyOptions().TenantKey;
@@ -26,7 +26,7 @@ namespace Volo.Abp.AspNetCore.MultiTenancy
             var tenantIdHeader = httpContext.Request.Headers[tenantIdKey];
             if (tenantIdHeader == string.Empty || tenantIdHeader.Count < 1)
             {
-                return null;
+                return Task.FromResult((string)null);
             }
 
             if (tenantIdHeader.Count > 1)

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

@@ -12,9 +12,9 @@ namespace Volo.Abp.AspNetCore.MultiTenancy
 
         protected override Task<string> GetTenantIdOrNameFromHttpContextOrNullAsync(ITenantResolveContext context, HttpContext httpContext)
         {
-            return httpContext.Request.QueryString.HasValue
-                ? Task.FromResult(httpContext.Request.Query[context.GetAbpAspNetCoreMultiTenancyOptions().TenantKey].ToString())
-                : null;
+            return Task.FromResult(httpContext.Request.QueryString.HasValue
+                ? httpContext.Request.Query[context.GetAbpAspNetCoreMultiTenancyOptions().TenantKey].ToString()
+                : null);
         }
     }
 }

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

@@ -15,7 +15,7 @@ namespace Volo.Abp.AspNetCore.MultiTenancy
         protected override Task<string> GetTenantIdOrNameFromHttpContextOrNullAsync(ITenantResolveContext context, HttpContext httpContext)
         {
             var tenantId = httpContext.GetRouteValue(context.GetAbpAspNetCoreMultiTenancyOptions().TenantKey);
-            return tenantId != null ? Task.FromResult(Convert.ToString(tenantId)) : null;
+            return Task.FromResult(tenantId != null ? Convert.ToString(tenantId) : null);
         }
     }
 }