Fix php warning when adding empty spf

If you add a SPF record without selecting any of the SPF options (mx, a, etc) the code tries to implode() on a NULL variable. We should always initialize this as an empty array.

[Thu Feb 27 14:17:51 2020] PHP Warning:  implode(): Invalid arguments passed in /private/tmp/ispconfig3_install/interface/web/dns/dns_spf_edit.php on line 235