MDL-58050 message: Commit the transaction before the early return

If there was no conversation found and the method returned early, the
active transaction was not marked as committable and the error was
reported by the request shutdown handler.

Attaching a unit test for the case just because it did not seem to be
covered. I was trying to reproduce the thrown error in the unit test but
it can't be used in this case. The shutdown handler puts the warning
directly into the error_log and it is not guaranteed where such messages
go (depending on the PHP configuration). And we do not even raise it
during the unit test execution (presumably due to noise it would
produce) anyway.

message/classes/api.php

@@ -314,6 +314,7 @@ class api {
 
         // This user has no conversations so we can return early here.
         if (empty($conversationrecords)) {
+            $transaction->allow_commit();
             return [];
         }
 

message/tests/api_test.php

@@ -333,6 +333,9 @@ class core_message_api_testcase extends core_message_messagelib_testcase {
         // The person doing the search.
         $this->setUser($user1);
 
+        // No conversations yet.
+        $this->assertEquals([], \core_message\api::get_conversations($user1->id));
+
         // Send some messages back and forth, have some different conversations with different users.
         $time = 1;
         $this->send_fake_message($user1, $user2, 'Yo!', 0, $time + 1);