Bulk selector should not be nullable, fix NRE in DebugInformation

Cherry-pick of #2054

Author: gmarz

src/Nest/CommonAbstractions/Response/ResponseBase.cs

@@ -43,16 +43,16 @@ public string DebugInformation
 			get
 			{
 				var sb = new StringBuilder();
-				sb.Append($"{(!IsValid ? "Inv" : "V")}alid NEST response built from a ");
-				sb.AppendLine(ApiCall?.ToString().ToCamelCase() ?? "null ApiCall which is highly exceptional, please open a bug if you see this");
+				sb.Append($"{(!this.IsValid ? "Inv" : "V")}alid NEST response built from a ");
+				sb.AppendLine(this.ApiCall?.ToString().ToCamelCase() ?? "null ApiCall which is highly exceptional, please open a bug if you see this");
 				if (!this.IsValid) this.DebugIsValid(sb);
-				ResponseStatics.DebugInformationBuilder(ApiCall, sb);
+				if (this.ApiCall != null) ResponseStatics.DebugInformationBuilder(ApiCall, sb);
 				return sb.ToString();
 			}
 		}
 		protected virtual void DebugIsValid(StringBuilder sb) { }
 
-		public override string ToString() =>  $"{(!IsValid ? "Inv" : "V")}alid NEST response built from a {this.ApiCall?.ToString().ToCamelCase()}";
+		public override string ToString() =>  $"{(!this.IsValid ? "Inv" : "V")}alid NEST response built from a {this.ApiCall?.ToString().ToCamelCase()}";
 
 	}
 }

src/Nest/Document/Multiple/Bulk/BulkResponse.cs

@@ -19,6 +19,7 @@ public class BulkResponse : ResponseBase, IBulkResponse
 		public override bool IsValid => base.IsValid && !this.Errors && !this.ItemsWithErrors.HasAny();
 		protected override void DebugIsValid(StringBuilder sb)
 		{
+			if (this.Items == null) return;
 			sb.AppendLine($"# Invalid Bulk items:");
 			foreach(var i in Items.Select((item, i) => new { item, i}).Where(i=>!i.item.IsValid))
 				sb.AppendLine($"  operation[{i.i}]: {i.item}");

src/Nest/Document/Multiple/Bulk/ElasticClient-Bulk.cs

@@ -7,7 +7,7 @@ namespace Nest
 	public partial interface IElasticClient
 	{
 		/// <summary>
-		/// The bulk API makes it possible to perform many index/delete operations in a single API call. 
+		/// The bulk API makes it possible to perform many index/delete operations in a single API call.
 		/// This can greatly increase the indexing speed.
 		/// <para> </para>http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/docs-bulk.html
 		/// </summary>
@@ -28,23 +28,23 @@ public partial interface IElasticClient
 	public partial class ElasticClient
 	{
 		/// <inheritdoc/>
-		public IBulkResponse Bulk(IBulkRequest request) => 
+		public IBulkResponse Bulk(IBulkRequest request) =>
 			this.Dispatcher.Dispatch<IBulkRequest, BulkRequestParameters, BulkResponse>(
 				request, this.LowLevelDispatch.BulkDispatch<BulkResponse>
 			);
 
 		/// <inheritdoc/>
-		public IBulkResponse Bulk(Func<BulkDescriptor, IBulkRequest> selector = null) =>
+		public IBulkResponse Bulk(Func<BulkDescriptor, IBulkRequest> selector) =>
 			this.Bulk(selector.InvokeOrDefault(new BulkDescriptor()));
 
 		/// <inheritdoc/>
-		public Task<IBulkResponse> BulkAsync(IBulkRequest request) => 
+		public Task<IBulkResponse> BulkAsync(IBulkRequest request) =>
 			this.Dispatcher.DispatchAsync<IBulkRequest, BulkRequestParameters, BulkResponse, IBulkResponse>(
 				request, this.LowLevelDispatch.BulkDispatchAsync<BulkResponse>
 			);
 
 		/// <inheritdoc/>
-		public Task<IBulkResponse> BulkAsync(Func<BulkDescriptor, IBulkRequest> selector = null) =>
+		public Task<IBulkResponse> BulkAsync(Func<BulkDescriptor, IBulkRequest> selector) =>
 			this.BulkAsync(selector.InvokeOrDefault(new BulkDescriptor()));
 	}
-}
+}