Codechange: use std::optional<std::string> over char * for text query result

[rubidium42]

Motivation / Problem

OnQueryTextFinished gets a char * which in many places gets passed into StrEmpty.

The char * fulfils multiple meanings; when it's nullptr it means the window was closed and there is no string, when the string is empty (str[0] == '\0') this denotes the default value (also functionally; clearing the company name and then pressing OK will give you the default name), and then anything non-empty is considered valid data.

Since we're moving towards a more C++-styled codebase, convert this char * into std::optional<std::string> instead.

Description

Convert the parameter type of OnQueryTextFinished.

Limitations

This introduces quite a few ->c_str()s because many places use atoi and friends. Replacing atoi with a more modern variant is beyond the scope of this PR.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[LordAro]

A few more const-ings? I don't think these are modified anywhere within the OnQuery* functions?

Not sure about & either, presumably the std::optional is already avoiding a copy?

[PeterN]

std::optional is not avoiding a copy.

[rubidium42]

std::optional doesn't allow references.

Then there is the choice whether we want to make everything const, or do we want the string to be std::moveable? There are three places where we could use std::move as data is written directly to variables. In all other cases it's either ->c_str() or passed as a const std::string & into a command.

[PeterN]

Who owned the string when it was char * form? std::string_view might be appropriate, but then again is also potentially as dangerous as char * in this context.

[rubidium42]

std::string_view requires changing many command signatures as well as move std::string_view much deeper into the settings. That would rabbit hole quite quickly.