[HttpKernel] Initialize DumpDataCollector data as array

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Without this I'm getting an exception when I open the debug panel without collected data.

```
ContextErrorException: Warning: Invalid argument supplied for foreach()   -
in vendor/symfony/symfony/src/Symfony/Component/HttpKernel/DataCollector/DumpDataCollector.php at line 165 

any reasons to make it protected instead of private?