Remove null check here because we already do it elsewhere

src/Workspaces/Core/Portable/Workspace/Solution/ProjectState.cs

@@ -673,11 +673,6 @@ public ProjectState UpdateAdditionalDocument(TextDocumentState newDocument, bool
 
         public ProjectState UpdateDocumentsOrder(ImmutableList<DocumentId> documentIds)
         {
-            if (documentIds == null)
-            {
-                throw new ArgumentNullException(nameof(documentIds));
-            }
-
             if (documentIds.IsEmpty)
             {
                 throw new ArgumentOutOfRangeException("The specified documents are empty.", nameof(documentIds));