Add null check to ReferenceCollection

Author: asbjornu

src/GitVersion.LibGit2Sharp/Git/ReferenceCollection.cs

@@ -1,9 +1,13 @@
+using GitVersion.Extensions;
+
 namespace GitVersion;
 
 internal sealed class ReferenceCollection : IReferenceCollection
 {
     private readonly LibGit2Sharp.ReferenceCollection innerCollection;
-    internal ReferenceCollection(LibGit2Sharp.ReferenceCollection collection) => this.innerCollection = collection;
+
+    internal ReferenceCollection(LibGit2Sharp.ReferenceCollection collection)
+        => this.innerCollection = collection.NotNull();
 
     public IEnumerator<IReference> GetEnumerator() => this.innerCollection.Select(reference => new Reference(reference)).GetEnumerator();