Improve readability in `./source/creational/builder.swift`

[VladimirCreator]

There is an opportunity to improve readability in ./source/creational/builder.swift. The opportunity is to use guard let... instead of if let... reducing indenting's length.

[ochococo]

Do we still want do fix it @VladimirCreator?

[VladimirCreator]

@ochococo Sure!