Do not allow null in observations columns

Having null values requires us to check for null instead of using ObservationCollection object. We should keep the values as {} and make it default value. Not making it default may cause the server to break, if null.

Analysis:
- Was created as result of https://github.com/avniproject/data-fixes/issues/2
- The data has become null because of data loss issue - date matches with created_date_time
- If null can be updated on column with default value then not null null constraint also needed
- Also need to check in setting place this check need to be made

The issue does not name files, tests, or an entry point. Start by locating the observations columns and the setting where their values are written, then trace how nulls and {} defaults are handled in the database and server. Done means the null-value and data-loss scenarios described here are covered and the server no longer breaks on null observations.

Written by the indexing model from the issue text.