Achieve 100% code/test coverage for `schema.ts`

[blackmann]

Currently https://github.com/blackmann/mangobase/blob/98d80085c450e83c6028fef98a9dd8cf3d2f5cec/base/src/schema.ts coverage by unit tests is 88%. Lets send it to 100%

Run yarn test:base src/schema.test.ts

Objectives

You'll see in red, line or line groups that have not been covered by the tests. Add tests in relevant describe blocks to cover those lines.