Throw DslException when no field for field value factor

Author: UdiBen

src/Nest/DSL/Query/FunctionScoreQueryDescriptor.cs

@@ -358,6 +358,8 @@ public FieldValueFactor(Action<FieldValueFactorDescriptor<T>> descriptorBuilder)
 		{
 			var descriptor = new FieldValueFactorDescriptor<T>();
 			descriptorBuilder(descriptor);
+			if (descriptor._Field.IsConditionless())
+				throw new DslException("Field name not set for field value factor function");
 
 			this._FieldValueFactor = descriptor;
 		}

src/Tests/Nest.Tests.Unit/Search/Query/Singles/FunctionScoreQueryJson.cs

@@ -86,5 +86,18 @@ public void FunctionScoreQueryConditionless()
 			}";
 			Assert.True(json.JsonEquals(expected), json);
 		}
+
+		[Test]
+		public void ConditionlessFieldValueFactor()
+		{
+			Assert.Throws<DslException>(() => new SearchDescriptor<ElasticsearchProject>().From(0).Size(10)
+				.Query(q => q
+					.FunctionScore(fs => fs
+						.Query(qq => qq.Term("", ""))
+						.Functions(
+							f => f.FieldValueFactor(db => db.Factor(3.4).Modifier(FieldValueFactorModifier.ln))
+						))
+				));
+		}
 	}
 }