Fix double evaluation of IEnumerable methods

The ToListOrNullIfEmpty() extension method called .Any() and .ToList()
which both evaluate the IEnumerable.  This caused funcs to be executed
twice when applied to an IEnumerable<Func<>>.

Closes #2101

Author: gmarz

src/Nest/CommonAbstractions/Extensions/Extensions.cs

@@ -161,8 +161,11 @@ internal static void ForEach<T>(this IEnumerable<T> enumerable, Action<T> handle
 			foreach (T item in enumerable) handler(item);
 		}
 
-		internal static List<T> ToListOrNullIfEmpty<T>(this IEnumerable<T> enumerable) =>
-			enumerable.HasAny() ? enumerable.ToList() : null;
+		internal static List<T> ToListOrNullIfEmpty<T>(this IEnumerable<T> enumerable)
+		{
+			var list = enumerable?.ToList();
+			return list.HasAny() ? list : null;
+		}
 
 		internal static void AddIfNotNull<T>(this IList<T> list, T item) where T : class
 		{

src/Tests/Reproduce/GithubIssue2101.cs

@@ -0,0 +1,51 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Elasticsearch.Net;
+using Nest;
+using FluentAssertions;
+using Tests.Framework;
+
+namespace Tests.Reproduce
+{
+	public class GithubIssue2101
+	{
+
+		[U]
+		public void BoolClausesShouldEvaluateOnlyOnce()
+		{
+			var must = 0;
+			var mustNot = 0;
+			var should = 0;
+			var filter = 0;
+
+			new BoolQueryDescriptor<object>()
+				.Must(m =>
+				{
+					must++;
+					return m;
+				})
+				.MustNot(mn =>
+				{
+					mustNot++;
+					return mn;
+				})
+				.Should(sh =>
+				{
+					should++;
+					return sh;
+				})
+				.Filter(f =>
+				{
+					filter++;
+					return f;
+				});
+
+			filter.Should().Be(1);
+			should.Should().Be(1);
+			must.Should().Be(1);
+			mustNot.Should().Be(1);
+		}
+	}
+}

src/Tests/Tests.csproj

@@ -570,6 +570,7 @@
     <Compile Include="Search\MultiSearch\MultiSearchApiTests.cs" />
     <Compile Include="Search\MultiSearch\MultiSearchInvalidApiTests.cs" />
     <Compile Include="QueryDsl\Verbatim\VerbatimAndStrictQueryUsageTests.cs" />
+    <Compile Include="Reproduce\GithubIssue2101.cs" />
     <Compile Include="Search\Search\Rescoring\RescoreUsageTests.cs" />
     <Compile Include="XPack\Security\ClearCachedRealms\ClearCachedRealmsApiTests.cs" />
     <Compile Include="XPack\Security\ClearCachedRealms\ClearCachedRealmsUrlTests.cs" />