Improve null value handling in settings.json instead of implicit string conversion

[coderabbitai]

Problem

Currently, the codebase relies on implicit conversion where ${null} == "null" when handling null values in settings.json. This approach is not robust and may cause issues in the future.

Context

In PR #1042, the code was changed to set archives_table_name and files_table_name to empty strings for CLP_S storage engine instead of using null values. While this works for the current implementation, it highlights the need for proper null value handling.

Proposed Solution

• Update settings.json typing to properly handle null values
• Implement proper null value handling instead of relying on implicit string conversion
• Consider how null values should be properly serialized and deserialized

References

• PR: fix(new-webui): Add support for querying metadata from multiple datasets (fixes #1024). #1042
• Comment: fix(new-webui): Add support for querying metadata from multiple datasets (fixes #1024). #1042 (comment)
• Requested by: hoophalab