Debug Validation Error Aggregation

You are given a Python validation library similar to Colander. A schema node can run multiple validators. Validators may raise validation errors, and all errors are aggregated into a structured dictionary through an `asdict()` method. The current implementation has three failing test groups. Your task is to debug the code, identify the root causes, and implement fixes. ### Requirements 1. **Handle null or empty validation messages** - Some validators produce `None`, an empty string, or lists containing null/empty messages. - Aggregation currently crashes when combining these messages. - Fix the aggregation logic so it filters invalid messages before combining them. - Real messages should still be preserved. 2. **Handle nested validation errors correctly** - Validators can be nested across multiple schema levels. - Errors from inner validations must be propagated and combined correctly in `asdict()`. - Nested structures may contain values such as `[None, 'must be positive']`. - The final output should not crash, should not include null messages, and should not lose valid child errors. 3. **Make sequence serialization defaults consistent** - During serialization, empty inputs and null-like values are currently handled differently. - For sequence fields, inputs such as `[]` and lists containing only null-like values should be treated consistently. - Defaults should be applied uniformly regardless of whether the sequence is empty or contains only null-like placeholders. Explain how you would debug the failures and what code-level changes you would make.