Support {} behavior for asset field accounts - Closes #3717

[michielmulders]

What was the problem?

Currently, the account field in accounts column holds null if we don't pass a JSON object for asset field.

How did I fix it?

Modify behavior to store empty object {}.

How to test it?

Run tests and check in DB if all accounts have {} under asset field.

Review checklist

• The PR resolves Modify asset behavior for accounts in Postgres #3717
• All new code is covered with unit tests
• All new code was formatted with Prettier
• Linting passes
• Tests pass
• Commit messages follow the commit guidelines
• Documentation has been added/updated

[lsilvs]

If the default value for asset is now {}, we need a migration to update the existing entries.

[michielmulders]

Or we include this in release/2.0?

Current migration ALTER TABLE "mem_accounts" ADD COLUMN IF NOT EXISTS "asset" jsonb NOT NULL DEFAULT '{}'::jsonb;

[MaciejBaj]

Shouldn't affect any block processing functions

[michielmulders]

@MaciejBaj no should not