bug #29981 [Security] Complain about an empty decision strategy (corphi)

This PR was merged into the 3.4 branch.

Discussion
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[Security] Complain about an empty decision strategy

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

When an empty string is passed (or objects with a similarly behaving `__toString()` method) to the constructor, the call to `decide` causes infinite recursion.

Commits
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3a22cad29b Fix infinite recursion when passed an empty string

src/Symfony/Component/Security/Core/Authorization/AccessDecisionManager.php

@@ -43,7 +43,7 @@ class AccessDecisionManager implements AccessDecisionManagerInterface
     public function __construct($voters = [], $strategy = self::STRATEGY_AFFIRMATIVE, $allowIfAllAbstainDecisions = false, $allowIfEqualGrantedDeniedDecisions = true)
     {
         $strategyMethod = 'decide'.ucfirst($strategy);
-        if (!\is_callable([$this, $strategyMethod])) {
+        if ('' === $strategy || !\is_callable([$this, $strategyMethod])) {
             throw new \InvalidArgumentException(sprintf('The strategy "%s" is not supported.', $strategy));
         }