MDL-77156 enrol_self: deprecated trim() of null in PHP 8.1

enrol/self/lib.php

@@ -403,7 +403,7 @@ protected function email_welcome_message($instance, $user) {
         $a->coursename = format_string($course->fullname, true, array('context'=>$context));
         $a->profileurl = "$CFG->wwwroot/user/view.php?id=$user->id&course=$course->id";
 
-        if (trim($instance->customtext1) !== '') {
+        if (!is_null($instance->customtext1) && trim($instance->customtext1) !== '') {
             $message = $instance->customtext1;
             $key = array('{$a->coursename}', '{$a->profileurl}', '{$a->fullname}', '{$a->email}');
             $value = array($a->coursename, $a->profileurl, fullname($user), $user->email);

enrol/self/tests/behat/self_enrolment.feature

@@ -140,3 +140,19 @@ Feature: Users can auto-enrol themself in courses where self enrolment is allowe
     And I navigate to "Unenrol me from C1" in current page administration
     And I click on "Continue" "button" in the "Confirm" "dialogue"
     Then I should see "You are unenrolled from the course \"Course 1\""
+
+  @javascript
+  Scenario: Self-enrolment enabled with simultaneous guest access
+    Given I log in as "teacher1"
+    And I am on the "Course 1" "enrolment methods" page
+    And I click on "Enable" "link" in the "Self enrolment (Student)" "table_row"
+    And I click on "Edit" "link" in the "Guest access" "table_row"
+    And I set the following fields to these values:
+      | Allow guest access | Yes |
+    And I press "Save changes"
+    And I log out
+    And I log in as "student1"
+    And I am on "Course 1" course homepage
+    And I navigate to "Enrol me in this course" in current page administration
+    And I click on "Enrol me" "button"
+    Then I should see "Topic 1"