fix(agc): skip an indexed draw with no indices or instances

[mrfelek]

What

A direct indexed draw with zero indices or zero instances threw zero-count indexed draws are unsupported. The auto (non-indexed) path already records nothing for such a draw. The indexed path now does the same: the draw has nothing to rasterize, so no work is recorded. Indirect draws are unchanged.

New case in agc_driver_graphics_tests: an indexed draw with zero indices and one with zero instances are skipped. The zero-index case fails without the change with "an indexed draw without indices was not skipped".

Tested

Ubuntu 26.10, GCC 15.3.0, RTX 3080 Ti (NVIDIA 615.71.09), Release, on main 146f171.

• agc_driver_graphics_tests passes; fails without the change.
• Depth tests that share the code pass: depth_plane_sampling, depth_storage, depth_plane_guest_layers, depth_storage_dispatch, depth_storage_reuse, depth_compare_copy.
• Diablo II: Resurrected (PPSA03912, 01.039.000): Offline > New Character aborted with this message. With the change the class selection screen draws and the game reaches the Rogue Encampment (very slow, see ASTRO BOT (PPSA21564): scene renders black — content-level rendering (presentation works) #3961). That run also had two local depth changes (see feat(agc): sample depth plane arrays from a later base slice #1186, feat(agc): storage image access to depth planes through 2D array views #5003).

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw; no silent stubs added
• No notes, investigation .md files or images added to the repository
• New third-party code is a submodule built from source (none)
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: PPSA03912 Diablo II: Resurrected (PPSA03912) #5146
• GPUs tested: NVIDIA GeForce RTX 3080 Ti, driver 615.71.09, Ubuntu 26.10
• AI-assisted: yes (Claude Code); I reviewed every line and ran the tests and the title