render.renderer.Renderer.getDmabufFormats() can return null

Tried testing a custom compositor which calls the method `render.renderer.getDmabufFormats()` (`wlr_renderer_get_dmabuf_texture_formats`) with `WLR_RENDERER=pixman` and that causes `getDmabufFormats` to return `null` but `render.drm_format_set.DrmFormatSet` cannot be used if it is `null`.