Make section start "nullable" (represented by std::nullopt)

[antis81]

The basic idea is to enable bass's SymbolTable to interpret nullopt (aka "undefined", "None" or "nil") for typed arguments. Currently such state does not exist (which in worst case can result in output holding "false positive" bytes - e.g. 0xff sometimes means -1 and sometimes it should error out instead - :zap:)

Addressing section "start", "pc" and "size" as a starting point.

To keep changes reviewable the logical behaviour of the assembler does not change here (at least not yet). What changes is only the comparison section.start == -1 vs !section.start.has_value() (alternatively section.start == std::nullopt).