bug symfony#31076 [HttpKernel] Fixed LoggerDataCollector crashing on empty file (althaus)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes symfony#31076).

Discussion
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[HttpKernel] Fixed LoggerDataCollector crashing on empty file

| Q             | A
| ------------- | ---
| Branch?       | 4.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#31050
| License       | MIT

This PR adds a check to the LoggerDataCollector if there's acutal some content in the log file.

Commits
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291c73a Catch empty deprecation.log silently (fixes symfony#31050)

Author: nicolas-grekas

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;