MDL-34851 show enrolment date if there is no start and end of active enrolment

This should help when enrol plugin does not set any start or end of enrolment validity.

enrol/locallib.php

@@ -863,7 +863,8 @@ class course_enrolment_manager {
                     $period = get_string('periodend', 'enrol', userdate($ue->timeend));
                     $periodoutside = ($ue->timeend && $now > $ue->timeend);
                 } else {
-                    $period = '';
+                    // If there is no start or end show when user was enrolled.
+                    $period = get_string('periodnone', 'enrol', userdate($ue->timecreated));
                     $periodoutside = false;
                 }
                 $details['enrolments'][$ue->id] = array(

lang/en/enrol.php

@@ -79,6 +79,7 @@ $string['participationactive'] = 'Active';
 $string['participationstatus'] = 'Status';
 $string['participationsuspended'] = 'Suspended';
 $string['periodend'] = 'until {$a}';
+$string['periodnone'] = 'enrolled {$a}';
 $string['periodstart'] = 'from {$a}';
 $string['periodstartend'] = 'from {$a->start} until {$a->end}';
 $string['recovergrades'] = 'Recover user\'s old grades if possible';