fix #1292 allow sort() on fluent search descriptor to return null to or no field/script set to prevent sort being sent

Author: Mpdreamz

src/Nest/DSL/SearchDescriptor.cs

@@ -680,6 +680,9 @@ public SearchDescriptor<T> Sort(Func<SortFieldDescriptor<T>, IFieldSort> sortSel
 
 			sortSelector.ThrowIfNull("sortSelector");
 			var descriptor = sortSelector(new SortFieldDescriptor<T>());
+			if (descriptor == null || descriptor.Field.IsConditionless())
+				return this;
+
 			Self.Sort.Add(new KeyValuePair<PropertyPathMarker, ISort>(descriptor.Field, descriptor));
 			return this;
 		}
@@ -695,6 +698,8 @@ public SearchDescriptor<T> SortGeoDistance(Func<SortGeoDistanceDescriptor<T>, IG
 
 			sortSelector.ThrowIfNull("sortSelector");
 			var descriptor = sortSelector(new SortGeoDistanceDescriptor<T>());
+			if (descriptor == null || descriptor.Field.IsConditionless())
+				return this;
 			Self.Sort.Add(new KeyValuePair<PropertyPathMarker, ISort>("_geo_distance", descriptor));
 			return this;
 		}
@@ -710,6 +715,8 @@ public SearchDescriptor<T> SortScript(Func<SortScriptDescriptor<T>, IScriptSort>
 
 			sortSelector.ThrowIfNull("sortSelector");
 			var descriptor = sortSelector(new SortScriptDescriptor<T>());
+			if (descriptor == null || descriptor.Script.IsNullOrEmpty())
+				return this;
 			Self.Sort.Add(new KeyValuePair<PropertyPathMarker, ISort>("_script", descriptor));
 			return this;
 		}