Handle a null string implicit conversion to DateMath

Use null conditional operator when attempting to cast terms params to object
Closes #1766

Author: russcam

Diff for: src/Nest/CommonOptions/DateMath/DateMath.cs

@@ -32,16 +32,18 @@ protected DateMath() { }
 		public static DateMathExpression Anchored(DateTime anchor) => new DateMathExpression(anchor);
 		public static DateMathExpression Anchored(string dateAnchor) => new DateMathExpression(dateAnchor);
 
-		private static Regex _dateMathRe =
+		private static readonly Regex _dateMathRe =
 			new Regex(@"^(?<anchor>now|.+(?:\|\||$))(?<ranges>(?:(?:\+|\-)[^\/]*))?(?<rounding>\/(?:y|M|w|d|h|m|s))?$");
 
 		public static implicit operator DateMath(DateTime dateTime) => DateMath.Anchored(dateTime);
 		public static implicit operator DateMath(string dateMath) => DateMath.FromString(dateMath);
 
 		public static DateMath FromString(string dateMath)
 		{
+			if (dateMath == null) return null;
+
 			var match = _dateMathRe.Match(dateMath);
-			if (!match.Success) throw new ArgumentException($"Can not create a DateMathExpression out of '{dateMath}'");
+			if (!match.Success) throw new ArgumentException($"Cannot create a DateMathExpression out of '{dateMath}'");
 
 			var math = new DateMathExpression(match.Groups["anchor"].Value);
 

Diff for: src/Nest/QueryDsl/TermLevel/Terms/TermsQuery.cs

@@ -28,7 +28,10 @@ internal static bool IsConditionless(ITermsQuery q)
 		{
 			return q.Field.IsConditionless() 
 				|| (
-				(!q.Terms.HasAny() || q.Terms.All(t=>t == null || ((t as string)?.IsNullOrEmpty()).GetValueOrDefault(false))
+				(q.Terms == null 
+					|| !q.Terms.HasAny() 
+					|| q.Terms.All(t=>t == null 
+					|| ((t as string)?.IsNullOrEmpty()).GetValueOrDefault(false))
 				)
 				&& 
 				(q.TermsLookup == null
@@ -63,9 +66,9 @@ public TermsQueryDescriptor<T, TValue> TermsLookup<TOther>(Func<FieldLookupDescr
 
 		public TermsQueryDescriptor<T, TValue> DisableCoord(bool? disable = true) => Assign(a => a.DisableCoord = disable);
 
-		public TermsQueryDescriptor<T, TValue> Terms(IEnumerable<TValue> terms) => Assign(a => a.Terms = terms.Cast<object>());
+		public TermsQueryDescriptor<T, TValue> Terms(IEnumerable<TValue> terms) => Assign(a => a.Terms = terms?.Cast<object>());
 
-		public TermsQueryDescriptor<T, TValue> Terms(params TValue[] terms) => Assign(a => a.Terms = terms.Cast<object>());
+		public TermsQueryDescriptor<T, TValue> Terms(params TValue[] terms) => Assign(a => a.Terms = terms?.Cast<object>());
 
 	}
 }

Diff for: src/Tests/CommonOptions/DateMath/DateMathTests.cs

@@ -0,0 +1,41 @@
+using System;
+using FluentAssertions;
+using Tests.Framework;
+
+namespace Tests.CommonOptions.DateMath
+{
+	public class DateMathTests
+	{
+		[U]
+		public void ImplicitConversionFromNullString()
+		{
+			string nullString = null;
+			Nest.DateMath dateMath = nullString;
+			dateMath.Should().BeNull();
+		}
+
+		[U]
+		public void ImplicitConversionFromNullNullableDateTime()
+		{
+			DateTime? nullableDateTime = null;
+			Nest.DateMath dateMath = nullableDateTime;
+			dateMath.Should().BeNull();
+		}
+
+		[U]
+		public void ImplicitConversionFromDateMathString()
+		{
+			string nullString = "now+3d";
+			Nest.DateMath dateMath = nullString;
+			dateMath.Should().NotBeNull();
+		}
+
+		[U]
+		public void ImplicitConversionFromNullableDateTimeWithValue()
+		{
+			DateTime? nullableDateTime = DateTime.Now;
+			Nest.DateMath dateMath = nullableDateTime;
+			dateMath.Should().NotBeNull();
+		}
+	}
+}

Diff for: src/Tests/Document/Single/Index/IndexApiTests.cs

@@ -194,7 +194,6 @@ public void Index()
 		{
 			var anonymousType = new
 			{
-				id = "id",
 				name = "name",
 				value = 3,
 				date = new DateTime(2016, 1, 1),
@@ -206,7 +205,7 @@ public void Index()
 			};
 
 			var indexResult = this.Client.Index(anonymousType, f => f
-				.Id(anonymousType.id)
+				.Id(anonymousType.name)
 			);
 
 			indexResult.IsValid.Should().BeTrue();