MDL-73350 qbank_statistics: Discrimination index column update

This implementation changes a little logic where the
discrimination index shows an empty row if there is
no attempts been made to a question rather than showing
very likely.

question/bank/statistics/classes/helper.php

@@ -204,7 +204,10 @@ class helper {
      * @return array
      */
     public static function format_discrimination_index(?float $value): array {
-        if (is_null($value) || $value < self::NEED_FOR_REVISION_LOWER_THRESHOLD) {
+        if (is_null($value)) {
+            $content = get_string('emptyvalue', 'qbank_statistics');
+            $classes = '';
+        } else if ($value < self::NEED_FOR_REVISION_LOWER_THRESHOLD) {
             $content = get_string('verylikely', 'qbank_statistics');
             $classes = 'alert-danger';
         } else if ($value < self::NEED_FOR_REVISION_UPPER_THRESHOLD) {

question/bank/statistics/lang/en/qbank_statistics.php

@@ -38,3 +38,4 @@ $string['verylikely'] = 'Very likely';
 $string['likely'] = 'Likely';
 $string['unlikely'] = 'Unlikely';
 $string['na'] = 'N/A';
+$string['emptyvalue'] = '-';

question/bank/statistics/version.php

@@ -26,6 +26,6 @@
 defined('MOODLE_INTERNAL') || die();
 
 $plugin->component = 'qbank_statistics';
-$plugin->version   = 2021092000;
+$plugin->version   = 2021121500;
 $plugin->requires  = 2021052500;
 $plugin->maturity  = MATURITY_STABLE;