Motivation / Problem

Since #9344, invalid NewGRFs could crash OpenTTD with a new spritegroup if both num_loading and num_loaded are zero.

Description

This is resolved by detecting this case and skipping creation of a sprite group.

Limitations

The invalid NewGRF may behave differently than before this (and #9344) however the NewGRF is still invalid, but it won't crash OpenTTD.

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, gs_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