bug symfony#20638 Fix legacy tests that do not trigger any depreciation (julienfalque)

This PR was merged into the 3.2 branch.

Discussion
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Fix legacy tests that do not trigger any depreciation

| Q             | A
| ------------- | ---
| Branch?       | 3.2
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Fixes `implode(): Invalid arguments passed` error when a `@legacy` test with `@expectedDeprecation` does not actually trigger any deprecation.

Commits
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9287b52 Fix tests that do not trigger any depreciation

Author: nicolas-grekas

src/Symfony/Bridge/PhpUnit/SymfonyTestsListener.php

@@ -25,8 +25,8 @@ class SymfonyTestsListener extends \PHPUnit_Framework_BaseTestListener
     private $skippedFile = false;
     private $wasSkipped = array();
     private $isSkipped = array();
-    private $expectedDeprecations;
-    private $gatheredDeprecations;
+    private $expectedDeprecations = array();
+    private $gatheredDeprecations = array();
     private $previousErrorHandler;
 
     /**
@@ -181,7 +181,8 @@ public function endTest(\PHPUnit_Framework_Test $test, $time)
                 $test->getTestResultObject()->addFailure($test, $e, $time);
             }
 
-            $this->expectedDeprecations = $this->gatheredDeprecations = $this->previousErrorHandler = null;
+            $this->expectedDeprecations = $this->gatheredDeprecations = array();
+            $this->previousErrorHandler = null;
         }
         if (-2 < $this->state && $test instanceof \PHPUnit_Framework_TestCase) {
             $groups = \PHPUnit_Util_Test::getGroups(get_class($test), $test->getName(false));