remove default null value for asset version

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.

Author: xabbuh

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; })