Use OfType instead of Where != null

src/Generation/GirTool/GenerateCommand.cs

@@ -143,8 +143,7 @@ private static DeserializedInput DeserializeInput(string platformName, string? s
 
         var inputRepositories = input
             .Select(fileName => repositoryResolver.ResolveRepository(fileName))
-            .Where(inputRepository => inputRepository != null)
-            .Select(inputRepository => inputRepository!)
+            .OfType<GirLoader.Input.Repository>()
             .ToList();
 
         // Get the namespaces corresponding to the input gir files.