bug symfony#24539 Fix LogLevel::DEBUG as min level (damner)

This PR was merged into the 4.0-dev branch.

Discussion
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Fix LogLevel::DEBUG as min level

| Q             | A
| ------------- | ---
| Branch?       | 3.4 or master <!-- see comment below -->
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md files -->
| Tests pass?   | yes
| Fixed tickets | <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | <!--highly recommended for new features-->

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Commits
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f15da80 Fix LogLevel::DEBUG as min level

Author: fabpot

src/Symfony/Component/HttpKernel/Log/Logger.php

@@ -39,7 +39,7 @@ class Logger extends AbstractLogger
 
     public function __construct($minLevel = null, $output = 'php://stderr', callable $formatter = null)
     {
-        if (!$minLevel) {
+        if (null === $minLevel) {
             $minLevel = LogLevel::WARNING;
 
             if (isset($_SERVER['SHELL_VERBOSITY'])) {