Improve debugging for https://github.com/flutter/flutter/issues/128680

In the linked issue, I cannot explain how we are getting a type error. However, this refactor eliminates the null check operator (by iterating over `MapEntries` rather than the keys) in hopes that there will be a more descriptive error in the future. The correctness of this change is verified by the existing tests in https://github.com/flutter/flutter/blob/master/packages/flutter_tools/test/general.shard/license_collector_test.dart