Fix InvalidOperationException when there are no full remote releases

src/Velopack/Internal/EnumerableExtensions.cs

@@ -105,7 +105,7 @@ namespace Velopack
             var result = new List<TSource>();
 
             using (var e = source.GetEnumerator()) {
-                if (!e.MoveNext()) throw new InvalidOperationException("Source sequence doesn't contain any elements.");
+                if (!e.MoveNext()) return result;
 
                 var current = e.Current;
                 var resKey = keySelector(current);

test/Velopack.Tests/UpdateManagerTests.cs

@@ -294,6 +294,18 @@ public class UpdateManagerTests
         Assert.Throws<ArgumentException>(() => um.CheckForUpdates());
     }
 
+    [Fact]
+    public void CheckFromEmptyFileSource()
+    {
+        using var logger = _output.BuildLoggerFor<UpdateManagerTests>();
+        using var _1 = Utility.GetTempDirectory(out var tempPath);
+        var source = new SimpleFileSource(new DirectoryInfo(tempPath));
+        var locator = new TestVelopackLocator("MyCoolApp", "1.0.0", tempPath, logger);
+        var um = new UpdateManager(source, null, logger, locator);
+        var info = um.CheckForUpdates();
+        Assert.Null(info);
+    }
+
     [Fact]
     public void NoUpdatesIfCurrentEqualsRemoteVersion()
     {