mesa: skip fallback draw call if no primitives are being drawn

../src/mesa/main/draw.c: In function ‘_mesa_draw_gallium_fallback’: ../src/mesa/main/draw.c:1056:4: warning: ‘prim’ may be used uninitialized [-Wmaybe-uninitialized] 1056 | ctx->Driver.Draw(ctx, prim, num_prims, index_size ? &ib : NULL, | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1057 | index_bounds_valid, info->primitive_restart, | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1058 | info->restart_index, min_index, max_index, | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1059 | info->instance_count, info->start_instance); | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~