MDL-76362 qtype_numerical: Support empty units for apply_units

question/type/numerical/questiontype.php

@@ -651,9 +651,13 @@ class qtype_numerical_answer_processor {
      *      by the unit multiplier, if any, and the unit string, for reference.
      */
     public function apply_units($response, $separateunit = null) {
+        if ($response === null || trim($response) === '') {
+            return [null, null, null];
+        }
+
         // Strip spaces (which may be thousands separators) and change other forms
         // of writing e to e.
-        $response = str_replace(' ', '', $response ?? '');
+        $response = str_replace(' ', '', $response);
         $response = preg_replace('~(?:e|E|(?:x|\*|×)10(?:\^|\*\*))([+-]?\d+)~', 'e$1', $response);
 
         // If a . is present or there are multiple , (i.e. 2,456,789 ) assume ,

question/type/numerical/tests/answerprocessor_test.php

@@ -62,7 +62,7 @@ class answerprocessor_test extends \advanced_testcase {
 
         $rc = new \ReflectionClass($ap);
         $rcm = $rc->getMethod('parse_response');
-        $rcm->setAccessible(True);
+        $rcm->setAccessible(true);
 
         $this->assertEquals($expected, $rcm->invoke($ap, $args));
     }
@@ -200,6 +200,9 @@ class answerprocessor_test extends \advanced_testcase {
 
             [1, 'frogs', null, '1 frogs'],
             [null, null, null, '. m/s'],
+            [null, null, null, null],
+            [null, null, null, ''],
+            [null, null, null, '    '],
         ];
     }