chore: hide grades when sub section dependents not available

[saeedbashir]

Description

LEARNER-10266

Hide the mid-term exam grading when descents aren't available and remove some warnings those were appearing in the file.

Before	After

[shafqat-muneer]

If we refactor it something like this:

    private func canDownloadAllSections(in chapter: CourseChapter) -> Bool {
        for sequential in chapter.childs where viewModel.sequentialsDownloadState[sequential.id] != nil {
            return true
        }
        return false
    }

Then we can avoid following two warnings:

Unused Optional Binding Violation: Prefer `!= nil` over `let _ =` (unused_optional_binding)

Prefer For-Where Violation: `where` clauses are preferred over a single `if` inside a `for` (for_where)

[shafqat-muneer]

LGTM 👍