Save attendance record set even if all are unknown

[radarhere]

Resolves #1042

An attendance record set is only saved when is not empty - in other words, when at least one person is not 'unknown'.

jethro-pmm/views/view_6_attendance__1_record.class.php

Lines 117 to 119 in d47be60

	if ($set->processForm($i)) {
	$set->save();
	}

jethro-pmm/db_objects/attendance_record_set.class.php

Lines 373 to 384 in d47be60

	function processForm($prefix)
	{
	$this->_attendance_records = Array();
	if (isset($_POST['attendances']) && isset($_POST['attendances'][$prefix])) {
	foreach ($_POST['attendances'][$prefix] as $personid => $present) {
	if (!empty($present) && ($present != 'unknown')) {
	$this->_attendance_records[$personid] = (int)($present == 'present');
	}
	}
	}
	return count($this->_attendance_records);
	}

However, the issue requests that it still save in that situation. This PR changes the behaviour to always save.