MDL-77917 communication_matrix: Topic must not be null

The Matrix space/room topic must always be a string. We should normalise
this on the way out of the API.

communication/provider/matrix/classes/matrix_rooms.php

@@ -136,9 +136,9 @@ class matrix_rooms {
     /**
      * Get the matrix room topic.
      *
-     * @return string|null
+     * @return string
      */
-    public function get_topic(): ?string {
-        return $this->record->topic;
+    public function get_topic(): string {
+        return $this->record->topic ?? '';
     }
 }

communication/provider/matrix/tests/matrix_rooms_test.php

@@ -49,6 +49,16 @@ class matrix_rooms_test extends \advanced_testcase {
     public function test_create_room_record(): void {
         $this->resetAfterTest();
 
+        $room = matrix_rooms::create_room_record(
+            processorid: 10000,
+            topic: null,
+        );
+        $this->assertInstanceOf(matrix_rooms::class, $room);
+        $this->assertEquals(10000, $room->get_processor_id());
+        $this->assertNotNull('', $room->get_topic());
+        $this->assertEquals('', $room->get_topic());
+        $this->assertNull($room->get_room_id());
+
         $room = matrix_rooms::create_room_record(
             processorid: 12345,
             topic: 'The topic of this room is thusly',