Refactor code to pass `no_implicit_optional` mypy rule

### Main idea

After updating Backend.AI's mypy version to 0.991 (#972), we can see bunch of errors complaining `Incompatible default for argument ...` error. As a temporary measure, we've overrided `implicit_optional` setting with `true` to disable corresponding. We now need to refactor our whole code so that we can pass type checking safely without overriding that config value.Maybe we can consider integrating https://github.com/hauntsaninja/no_implicit_optional with pants' `fmt` command so that invalid type definitions can be automatically fixed.

### Alternative ideas

_No response_

### Anything else?

_No response_

JIRA Issue: BA-224