If there are no enumerators, it enters an infinite loop. (#257)

* If there are no enumerators, it enters an infinite loop.

* Add test

src/SIL.Machine/Utils/EnumerableExtensions.cs

@@ -13,6 +13,8 @@ Func<IEnumerable<TSource>, TResult> selector
         {
             // ToList is necessary to avoid deferred execution
             List<IEnumerator<TSource>> enumerators = source.Select(seq => seq.GetEnumerator()).ToList();
+            if (enumerators.Count == 0)
+                yield break;
             try
             {
                 while (true)

tests/SIL.Machine.Tests/Utils/EnumberableExtensionTests.cs

@@ -0,0 +1,23 @@
+using NUnit.Framework;
+
+namespace SIL.Machine.Utils;
+
+[TestFixture]
+public class EnumerableExtensionTests
+{
+    [Test]
+    public void ZipMany_None()
+    {
+        var seqs = new List<IEnumerable<int>>();
+        IEnumerable<int> result = seqs.ZipMany(x => x.Sum());
+        Assert.That(result, Is.Empty);
+    }
+
+    [Test]
+    public void ZipMany_Two()
+    {
+        var seqs = new List<IEnumerable<int>> { new[] { 1, 2, 3 }, new[] { 4, 5, 6 } };
+        IEnumerable<int> result = seqs.ZipMany(x => x.Sum());
+        Assert.That(result, Is.EqualTo(new[] { 5, 7, 9 }));
+    }
+}