bug symfony#17605 [FrameworkBundle] remove default null value for asset version (xabbuh)

This PR was merged into the 2.7 branch.

Discussion
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[FrameworkBundle] remove default null value for asset version

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

Setting `null` as the version of a package means that it uses the empty
version strategy. However, omitting the `version` option entirely was
meant to fall back to the default version strategy. This is not possible
when the default version value is `null` as there is no way to remove
it.

Commits
-------

25f735f remove default null value for asset version

Author: fabpot

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -558,7 +558,6 @@ private function addAssetsSection(ArrayNodeDefinition $rootNode)
                                 ->fixXmlConfig('base_url')
                                 ->children()
                                     ->scalarNode('version')
-                                        ->defaultNull()
                                         ->beforeNormalization()
                                         ->ifTrue(function ($v) { return '' === $v; })
                                         ->then(function ($v) { return; })