Treat IngressRuleBuilder.Service as non-null

src/Microsoft.Tye.Core/ApplicationFactory.cs

@@ -265,7 +265,7 @@ namespace Microsoft.Tye
                         {
                             Host = configRule.Host,
                             Path = configRule.Path,
-                            Service = configRule.Service,
+                            Service = configRule.Service!, // validated elsewhere
                         };
                         ingress.Rules.Add(rule);
                     }

src/Microsoft.Tye.Core/ConfigModel/ConfigIngressRule.cs

@@ -2,12 +2,16 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.ComponentModel.DataAnnotations;
+
 namespace Microsoft.Tye.ConfigModel
 {
     public class ConfigIngressRule
     {
         public string? Path { get; set; }
         public string? Host { get; set; }
+
+        [Required]
         public string? Service { get; set; }
     }
 }

src/Microsoft.Tye.Core/IngressRuleBuilder.cs

@@ -8,6 +8,6 @@ namespace Microsoft.Tye
     {
         public string? Path { get; set; }
         public string? Host { get; set; }
-        public string? Service { get; set; }
+        public string Service { get; set; } = default!;
     }
 }