Fix build: add null checks to filtered query before checking IsConditionless

Author: gmarz

Diff for: src/Nest/DSL/Query/QueryDescriptor.cs

@@ -466,10 +466,10 @@ public QueryContainer Filtered(Action<FilteredQueryDescriptor<T>> selector)
 
 			var filtered = query as IFilteredQuery;
 
-			if (filtered.Query.IsConditionless)
+			if (filtered.Query != null && filtered.Query.IsConditionless)
 				filtered.Query = null;
 
-			if (filtered.Filter.IsConditionless)
+			if (filtered.Filter != null && filtered.Filter.IsConditionless)
 				filtered.Filter = null;
 
 			return this.New(query, q => q.Filtered = query);

Diff for: src/Tests/Nest.Tests.Unit/Search/Filter/Singles/TermsFilterJson.cs

@@ -165,8 +165,7 @@ public void TermsFilterWithNullTerms()
 							  filtered: {
 							  query: {
 							    match_all: {}
-							  },
-							  filter: {}
+							  }
 							}
 						  }";
 			Assert.True(json.JsonEquals(expected), json);