strv: Make sure strv_make_nulstr() always returns a valid nulstr
    
    strv_make_nulstr() is documented to always return a valid nulstr,
    but if the input is `NULL` we return a string terminated with only
    a single NUL terminator, so let's fix that and always terminate the
    resulting string with two NUL bytes.
    
    (cherry picked from commit 5ea173a91b2093664a9ebb9add678edd6f5d1efd)