Catch empty deprecation.log silently (fixes symfony#31050)

Author: althaus

src/Symfony/Component/HttpKernel/DataCollector/LoggerDataCollector.php

@@ -126,9 +126,13 @@ private function getContainerDeprecationLogs()
             return [];
         }
 
+        if ('' === $logContent = trim(file_get_contents($file))) {
+            return [];
+        }
+
         $bootTime = filemtime($file);
         $logs = [];
-        foreach (unserialize(file_get_contents($file)) as $log) {
+        foreach (unserialize($logContent) as $log) {
             $log['context'] = ['exception' => new SilencedErrorContext($log['type'], $log['file'], $log['line'], $log['trace'], $log['count'])];
             $log['timestamp'] = $bootTime;
             $log['priority'] = 100;