fix: skip printing unset variable

Signed-off-by: Lucas Larson <LucasLarson@riseup.net>

update

@@ -1,5 +1,5 @@
 #!/usr/bin/env sh
-update='2024-05-18'
+update='2024-06-19'
 command -p -- printf -- '\n\n                 .___       __\n __ ________   __\174 _\057____ _\057  \174_  ____\n\174  \174  \134____ \134 \057 __ \174\134__  \134\134   __\134\057 __ \134\n\174  \174  \057  \174_\076 \076 \057_\057 \174 \057 __ \134\174  \174 \134  ___\057\n\174____\057\174   __\057\134____ \174\050____  \057__\174  \134___  \076\n      \174__\174        \134\057     \134\057          \134\057\n a Lucas Larson production\n\n' >&2 && command -p -- sleep 1
 command -p -- printf -- '\360\237\223\241  verifying network connectivity' >&2
 command -p -- sleep 1
@@ -127,5 +127,5 @@ fi
 if command -v -- omz >/dev/null 2>&1; then omz update 2>/dev/null; fi
 if command -v -- rehash >/dev/null 2>&1; then rehash; fi
 unset -v -- update 2>/dev/null || update=''
-command -p -- printf -- '\n\342%s\234\205  update complete\n' "${update-}" >&2
+command -p -- printf -- '\n\342\234\205  update complete\n' >&2
 case "${SHELL##*/}" in *"${0##*-}") return ;; *) exit ;; esac