Add null check to RefSpecCollection

Author: asbjornu

src/GitVersion.LibGit2Sharp/Git/RefSpecCollection.cs

@@ -1,9 +1,14 @@
+using GitVersion.Extensions;
+
 namespace GitVersion;
 
 internal sealed class RefSpecCollection : IRefSpecCollection
 {
     private readonly LibGit2Sharp.RefSpecCollection innerCollection;
-    internal RefSpecCollection(LibGit2Sharp.RefSpecCollection collection) => this.innerCollection = collection;
+
+    internal RefSpecCollection(LibGit2Sharp.RefSpecCollection collection)
+        => this.innerCollection = collection.NotNull();
+
     public IEnumerator<IRefSpec> GetEnumerator() => this.innerCollection.Select(tag => new RefSpec(tag)).GetEnumerator();
     IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 }