bug #17514 [2.7][Asset] Add defaultNull to version configuration (ewgRa)

This PR was merged into the 2.7 branch.

Discussion
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[2.7][Asset] Add defaultNull to version configuration

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #14832, #16511
| License       | MIT
| Doc PR        |

In #16511 PR was omitted defaultNull version for one case.

Commits
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65adb72 add defaultNull to version

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

@@ -554,6 +554,7 @@ class Configuration implements ConfigurationInterface
                                 ->fixXmlConfig('base_url')
                                 ->children()
                                     ->scalarNode('version')
+                                        ->defaultNull()
                                         ->beforeNormalization()
                                         ->ifTrue(function ($v) { return '' === $v; })
                                         ->then(function ($v) { return; })