bug #40019 [ErrorHandler] Fix strpos error when trying to call a method without a name (Deuchnord)

This PR was squashed before being merged into the 4.4 branch.

Discussion
----------

[ErrorHandler] Fix strpos error when trying to call a method without a name

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | None (direct fix)
| License       | MIT
| Doc PR        | None
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When running the following code:

```php
class Foo
{
    // Some code here
}

$str = ''; // this should not happen, but for some reason, it did.
$foo->{$str}();
```

a fatal error occurs because the method name to execute is empty, but Symfony's error enhancer fails to parse it:

![Error screenshot](https://user-images.githubusercontent.com/7600265/106108704-ec019b80-6148-11eb-82bc-f7801e30fea4.png)

In this PR, I propose a fix with a more clear error to inform the developer about what happened.

Commits
-------

66be87bffc [ErrorHandler] Fix strpos error when trying to call a method without a name

src/Symfony/Component/Debug/FatalErrorHandler/UndefinedMethodFatalErrorHandler.php

@@ -40,7 +40,7 @@ class UndefinedMethodFatalErrorHandler implements FatalErrorHandlerInterface
 
         $message = sprintf('Attempted to call an undefined method named "%s" of class "%s".', $methodName, $className);
 
-        if (!class_exists($className) || null === $methods = get_class_methods($className)) {
+        if ('' === $methodName || !class_exists($className) || null === $methods = get_class_methods($className)) {
             // failed to get the class or its methods on which an unknown method was called (for example on an anonymous class)
             return new UndefinedMethodException($message, $exception);
         }

src/Symfony/Component/Debug/Tests/FatalErrorHandler/UndefinedMethodFatalErrorHandlerTest.php

@@ -48,6 +48,15 @@ class UndefinedMethodFatalErrorHandlerTest extends TestCase
                 ],
                 'Attempted to call an undefined method named "what" of class "SplObjectStorage".',
             ],
+            [
+                [
+                    'type' => 1,
+                    'line' => 12,
+                    'file' => 'foo.php',
+                    'message' => 'Call to undefined method SplObjectStorage::()',
+                ],
+                'Attempted to call an undefined method named "" of class "SplObjectStorage".',
+            ],
             [
                 [
                     'type' => 1,

src/Symfony/Component/ErrorHandler/ErrorEnhancer/UndefinedMethodErrorEnhancer.php

@@ -39,7 +39,7 @@ class UndefinedMethodErrorEnhancer implements ErrorEnhancerInterface
 
         $message = sprintf('Attempted to call an undefined method named "%s" of class "%s".', $methodName, $className);
 
-        if (!class_exists($className) || null === $methods = get_class_methods($className)) {
+        if ('' === $methodName || !class_exists($className) || null === $methods = get_class_methods($className)) {
             // failed to get the class or its methods on which an unknown method was called (for example on an anonymous class)
             return new UndefinedMethodError($message, $error);
         }

src/Symfony/Component/ErrorHandler/Tests/ErrorEnhancer/UndefinedMethodErrorEnhancerTest.php

@@ -40,6 +40,10 @@ class UndefinedMethodErrorEnhancerTest extends TestCase
                 'Call to undefined method SplObjectStorage::what()',
                 'Attempted to call an undefined method named "what" of class "SplObjectStorage".',
             ],
+            [
+                'Call to undefined method SplObjectStorage::()',
+                'Attempted to call an undefined method named "" of class "SplObjectStorage".',
+            ],
             [
                 'Call to undefined method SplObjectStorage::walid()',
                 "Attempted to call an undefined method named \"walid\" of class \"SplObjectStorage\".\nDid you mean to call \"valid\"?",