test(headless): skip lvgl-source assertions when the submodule is absent
release.yml's validate-shell job checks out without submodules, so the four tests that sed lib/lvgl/src/drivers/sdl/*.c failed with exit 2 and aborted the v0.99.101 release before any build ran. Assert the patch files first (they are the source of truth for what ships), then skip the working-tree half when the submodule is not checked out, matching the existing libhv-test convention.