render: ensure wlr_render_rect_options->box is nonempty

This optimization also fixes an validation error with the Vulkan renderer by ensuring vkCmdClearAttachments does not receive empty regions.

This is an alternative to !4281 (closed), which is a bit more general.

The old wlr_renderer drawing API also had an empty rect check, at render/wlr_renderer.c#L167.