Author: David Woloszyn
MDL-77840 mnet: Avoid passing nulls to base64_encode
http://git.moodle.org/gw?p=moodle.git;a=commit;h=88496af8502706d4a8aa0cbf201ae1ba2dec07d9
http://github.com/moodle/moodle/commit/88496af8502706d4a8aa0cbf201ae1ba2dec07d9
+ 4.1 en [opensslsignerror,core_error]
+ 4.1 en [opensslsealerror,core_error]