Do not reference unset env variables

While working on #40026 (closed) I stumbled over (other) errors in our tor-browser build log:

tar: Substituting `.' for empty member name
tar: : Cannot stat: No such file or directory
tar: Substituting `.' for empty member name
tar: : Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors

It turns out that tar does not like "${EXCLUDE_ELECTRUM}" "${EXCLUDE_NCPROP279}" being unset in the Linux case (doing a EXCLUDE_ELECTRUM="" does not set the env variable), yet still referencing them.

It seems those tar errors are not fatal but we should fix them as they make it harder to reason about our build log.