Fix script environment variable setting for empty lists

Fix script environment variable setting for empty lists

When setting an environment variable as a space-separated list, and
the list is empty, we must not delete the '=' before the value.

In practice putenv() is likely to discard the invalid string, leaving
the variable unset, but this is not guaranteed.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>