replace `isset` with coalesce operator (#1761)

src/Monolog/Processor/IntrospectionProcessor.php

@@ -97,11 +97,11 @@ class IntrospectionProcessor implements ProcessorInterface
         $record->extra = array_merge(
             $record->extra,
             [
-                'file'      => isset($trace[$i - 1]['file']) ? $trace[$i - 1]['file'] : null,
-                'line'      => isset($trace[$i - 1]['line']) ? $trace[$i - 1]['line'] : null,
-                'class'     => isset($trace[$i]['class']) ? $trace[$i]['class'] : null,
-                'callType'  => isset($trace[$i]['type']) ? $trace[$i]['type'] : null,
-                'function'  => isset($trace[$i]['function']) ? $trace[$i]['function'] : null,
+                'file'      => $trace[$i - 1]['file'] ?? null,
+                'line'      => $trace[$i - 1]['line'] ?? null,
+                'class'     => $trace[$i]['class'] ?? null,
+                'callType'  => $trace[$i]['type'] ?? null,
+                'function'  => $trace[$i]['function'] ?? null,
             ]
         );