fix #1980 GetAsync fails when specifying a null index.

```csharp
var response = await elasticClient.GetAsync<T>(id, s =>
  s.Index(null));
```

Would not default index back to inferred type of T.

This PR makes specifying null for required route values a noop

For optional routevalues null still clears the routevalue so the
following still works

```csharp
c => c.Index(project, i => i.Id(null))
```

will reset the id on and send the index request to
`/inferredindex/inferredtype`

Author: Mpdreamz

Diff for: src/CodeGeneration/CodeGeneration.LowLevelClient/Domain/CsharpMethod.cs

@@ -240,16 +240,18 @@ public IEnumerable<FluentRouteSetter> GetFluentRouteSetters()
 					paramName = paramName.ToLowerInvariant();
 
 				var routeValue = paramName;
+				var routeSetter = p.Required ? "Required" : "Optional";
+
 				if (paramName == "metric") routeValue = "(Metrics)metric";
 				else if (paramName == "indexMetric") routeValue = "(IndexMetrics)indexMetric";
 
-				var code = $"public {returnType} {p.InterfaceName}({ClrParamType(p.ClrTypeName)} {paramName}) => Assign(a=>a.RouteValues.Optional(\"{p.Name}\", {routeValue}));";
+				var code = $"public {returnType} {p.InterfaceName}({ClrParamType(p.ClrTypeName)} {paramName}) => Assign(a=>a.RouteValues.{routeSetter}(\"{p.Name}\", {routeValue}));";
 				var xmlDoc = $"///<summary>{p.Description}</summary>";
 				setters.Add(new FluentRouteSetter { Code = code, XmlDoc = xmlDoc });
 				if (paramName == "index" || paramName == "type")
 				{
 					code = $"public {returnType} {p.InterfaceName}<TOther>() where TOther : class ";
-					code += $"=> Assign(a=>a.RouteValues.Optional(\"{p.Name}\", ({p.ClrTypeName})typeof(TOther)));";
+					code += $"=> Assign(a=>a.RouteValues.{routeSetter}(\"{p.Name}\", ({p.ClrTypeName})typeof(TOther)));";
 					xmlDoc = $"///<summary>{p.Description}</summary>";
 					setters.Add(new FluentRouteSetter { Code = code, XmlDoc = xmlDoc });
 				}
@@ -268,7 +270,7 @@ public IEnumerable<FluentRouteSetter> GetFluentRouteSetters()
 				if (paramName == "fields" && p.Type == "list")
 				{
 					code = $"public {returnType} Fields<T>(params Expression<Func<T, object>>[] fields) ";
-					code += "=> Assign(a => a.RouteValues.Optional(\"fields\", (Fields)fields));";
+					code += $"=> Assign(a => a.RouteValues.{routeSetter}(\"fields\", (Fields)fields));";
 					xmlDoc = $"///<summary>{p.Description}</summary>";
 					setters.Add(new FluentRouteSetter { Code = code, XmlDoc = xmlDoc });
 				}

Diff for: src/Nest/CommonAbstractions/Infer/DocumentPath/DocumentPath.cs

@@ -37,13 +37,15 @@ public DocumentPath(Id id)
 
 		public DocumentPath<T> Index(IndexName index)
 		{
+			if (index == null) return this;
 			Self.Index = index;
 			return this;
 		}
 		public DocumentPath<T> Type(TypeName type)
 		{
+			if (type == null) return this;
 			Self.Type = type;
 			return this;
 		}
 	}
-}
+}

Diff for: src/Nest/CommonAbstractions/Request/RouteValues.cs

@@ -35,12 +35,13 @@ private string GetResolved(string route)
 
 		private RouteValues Route(string name, IUrlParameter routeValue, bool required = true)
 		{
-			if (routeValue == null)
+			if (routeValue == null && !required)
 			{
 				if (this._routeValues.ContainsKey(name))
 					this._routeValues.Remove(name);
 				return this;
 			}
+			else if (routeValue == null) return this;
 
 			this._routeValues[name] = routeValue;
 			return this;