Added tests for FilesyncCommand - #30

Description
-----------

This fixes #30, Installing Contao 4.6 with `symfony/dependency-injection:^3.0` would then be possible.

The only downside is that exceptions would no longer be logged if Contao 4.6 is installed with Symfony HTTP kernel 3.4 because logging was removed in e285e1ddef3d696d8eb5c38ac94cbbca302b432b.

The requirement for `symfony/monolog-bridge:4.1.*` can stay as it is I think, because it is not closely tied to other Symfony packages.

Commits
-------

8f7a29e6 Relax symfony HTTP kernel requirement
cc9ec784 Revert "Remove exception logging in the pretty error listener now that Symfony logs exceptions by default"

This reverts commit e285e1ddef3d696d8eb5c38ac94cbbca302b432b.
97e549d6 Only log if kernel version lower than 4.1.0
1788839b Fix the coding style
b3c1f690 Remove a phpDoc comment

Description
-----------

We encountered an `ErrorException` when the page of external redirect type has an insert tag that returns an empty string.

The problem in fact happens in the `UrlHelper` component of Symfony, but I think we should fix it on our side nevertheless.

Stack trace:

```
ErrorException: Warning: Uninitialized string offset 0
#36 /vendor/symfony/http-foundation/UrlHelper.php(42): Symfony\Component\HttpFoundation\UrlHelper::getAbsoluteUrl
#35 /vendor/contao/core-bundle/src/Routing/Content/StringResolver.php(40): Contao\CoreBundle\Routing\Content\StringResolver::resolve
#34 /vendor/contao/core-bundle/src/Routing/ContentUrlGenerator.php(150): Contao\CoreBundle\Routing\ContentUrlGenerator::resolveContent
#33 /vendor/contao/core-bundle/src/Routing/ContentUrlGenerator.php(161): Contao\CoreBundle\Routing\ContentUrlGenerator::resolveContent
#32 /vendor/contao/core-bundle/src/Routing/ContentUrlGenerator.php(70): Contao\CoreBundle\Routing\ContentUrlGenerator::generate
#31 /vendor/contao/core-bundle/contao/modules/Module.php(349): Contao\Module::renderNavigation
#30 /vendor/contao/core-bundle/contao/modules/Module.php(326): Contao\Module::renderNavigation
#29 /vendor/contao/core-bundle/contao/modules/Module.php(326): Contao\Module::renderNavigation
#28 /vendor/contao/core-bundle/contao/modules/ModuleNavigation.php(82): Contao\ModuleNavigation::compile
#27 /vendor/contao/core-bundle/contao/modules/Module.php(213): Contao\Module::generate
#26 /vendor/contao/core-bundle/contao/modules/ModuleNavigation.php(45): Contao\ModuleNavigation::generate
#25 /vendor/contao/core-bundle/contao/library/Contao/Controller.php(410): Contao\Controller::getFrontendModule
…
#1 /vendor/friendsofsymfony/http-cache/src/SymfonyCache/EventDispatchingHttpCache.php(96): Contao\ManagerBundle\HttpKernel\ContaoCache::handle
#0 /public/index.php(42): null
```

Commits
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756336e7 Fix a potential issue if the insert tags return an empty string in th…
0a9330c2 Throw an exception if the insert tag returns an empty value
46de08ef CS + Unit test
89521539 Rename the test method