FIX Don't write log buffer without db

This can happen during unit test execution.
I've removed the manual flushing in d0acfad which caused this.

But regardless of this change, there could be additional
messages added to the logger after this manual flush
which then result in the same situation.
So the safest way is to check the log handler
is actually in a state to write log messages.

This will now "swallow" lowlevel errors such as "no database selected",
but *only* for the buffer handler writing to the job database record.
Since it bubbles by default, other handlers will still handle that error,
e.g. outputting it to php://stdout.

src/Services/QueuedJobHandler.php

@@ -4,6 +4,7 @@
 
 use Monolog\Handler\AbstractProcessingHandler;
 use SilverStripe\Core\Injector\Injectable;
+use SilverStripe\ORM\DB;
 use Symbiote\QueuedJobs\DataObjects\QueuedJobDescriptor;
 
 /**
@@ -56,6 +57,12 @@ protected function write(array $record)
 
     public function handleBatch(array $records)
     {
+        // Guard against handlers auto-flushing on destruct.
+        // When this is part of a PHPUnit run, the connection to the database can have already been closed.
+        if (!DB::is_active()) {
+            return;
+        }
+
         foreach ($records as $record) {
             $this->job->addMessage($record['message'], $record['level_name'], $record['datetime']);
         };