[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 

It comes from the parent class.
On Sep 30, 2014 9:02 AM, "Fabien Potencier" <notifications@github.com>
wrote:

> In src/Symfony/Component/HttpKernel/DataCollector/DumpDataCollector.php:
>
> > @@ -24,6 +24,7 @@
> >   */
> >  class DumpDataCollector extends DataCollector implements DataDumperInterface
> >  {
> > +    protected $data = array();
>
> any reasons to make it protected instead of private?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/symfony/symfony/pull/12079/files#r18201612>.
>