Fatal error usually doesn't contain "file" and "line" fields in a sta…

…cktrace. This causes Monolog\Formatter\NormalizerFormatter to return normalized error without "trace" field at all. But code expects it. Fix of this behavior. Check monolog related issue - Seldaek/monolog#1736

src/Formatter/LogstashFormatter.php

@@ -80,7 +80,7 @@ protected function normalizeException(\Throwable $e, int $depth = 0)
     {
         $traceArray = parent::normalizeException($e, $depth);
 
-        if ($this->traceDepth !== 0) {
+        if ($this->traceDepth !== 0 && isset($traceArray['trace']) && is_array($traceArray['trace'])) {
             $traceArray['trace'] = array_slice($traceArray['trace'], 0, $this->traceDepth);
         }