Codechange: replace NULL with nullptr

[rubidium42]

Motivation / Problem

Someone commenting on NULL in a diff, made me wonder if there's still any NULL left. There is.

Description

Replace NULL with nullptr, in two commits; one for squirrel and one for the rest.

Limitations

There are still some NULLs in the code, but that's in printf output, comments and json.hpp defines.

./saveload/compat/cheat_sl_compat.h:26: SLC(1, SL_MIN_VERSION, SLV_TABLE_CHUNKS), // Need to be two NULL fields. See Load_CHTS().
./saveload/compat/cheat_sl_compat.h:30: SLC(1, SL_MIN_VERSION, SLV_TABLE_CHUNKS), // Need to be two NULL fields. See Load_CHTS().
./saveload/compat/cheat_sl_compat.h:36: SLC(1, SL_MIN_VERSION, SLV_TABLE_CHUNKS), // Need to be two NULL fields. See Load_CHTS().
./saveload/saveload.h:708:      uint16_t length;                ///< Length of the NULL field.
./saveload/saveload.cpp:310:     * during NULLing; especially those that try to get
./stdafx.h:145:#        pragma warning(disable: 6011)   // code analyzer: Dereferencing NULL pointer 'pfGetAddrInfo': Lines: 995, 996, 998, 999, 1001
./fontcache.cpp:67:             return "[NULL]";
./3rdparty/squirrel/squirrel/sqvm.cpp:1583:             case OT:                        printf("NULL"); break;
./3rdparty/squirrel/sqstdlib/sqstdaux.cpp:56:                                   pf(v,fmt::format("[{}] NULL\n",name));
./3rdparty/nlohmann/json.hpp:2198:    #elif defined(NULL)
./3rdparty/nlohmann/json.hpp:2199:        #define JSON_HEDLEY NULL
./3rdparty/nlohmann/json.hpp:2203:#elif defined(NULL)
./3rdparty/nlohmann/json.hpp:2204:    #define JSON_HEDLEY NULL
./3rdparty/catch2/catch.hpp:2314:    // Specialised comparison functions to handle equality comparisons between ints and pointers (NULL deduces as an int)

The Microsoft API uses NULL, which also allows nullptr. We were already using nullptr in some places so I made it consistent.

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

[TrueBrain]

Sure.