[linter, DAS] Makes use_null_aware_elements to report on cascade elements

Fixes: https://github.com/dart-lang/sdk/issues/62660
Change-Id: I8daee991353cae128ea84cf8d71ee430a884f550
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/480560
Reviewed-by: Nate Biggs <natebiggs@google.com>
Auto-Submit: Felipe Morschel <git@fmorschel.dev>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>

pkg/dartdev/lib/src/templates.dart

@@ -100,7 +100,7 @@ abstract class Generator implements Comparable<Generator> {
       'description': description,
       'year': DateTime.now().year.toString(),
       'author': '<your name>',
-      if (additionalVars != null) ...additionalVars,
+      ...?additionalVars,
     };
 
     for (TemplateFile file in files) {