rustup-init.sh: Fix unset variable usage
rustup-init.sh: Fix unset variable usage

With `set -u` our check for `RUSTUP_UPDATE_ROOT` being empty
needed an additional tweak for it being unset.

Signed-off-by: Daniel Silverstone <3cc17a76d780e0cd3d87bad586a1c53967be5625@digital-scurf.org>

inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,inejge/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs