On a production machine, the Git last push widget image is correctly displayed.

Reproduction:
- In a project without any Git repo, add the last Git push widget
- The image can not be displayed by the browser

Stack trace:
Warning: Invalid argument supplied for foreach() in /usr/share/tuleap/plugins/git/include/Git_LastPushesGraph.class.php on line 132
Call Stack
# Time Memory Function Location
1 0.0000 636760 {main}( ) ../index.php:0
2 0.0441 5329608 GitPlugin->process( ) ../index.php:39
3 0.0524 6563560 PluginController->process( ) ../gitPlugin.class.php:471
4 0.0524 6563560 Git->request( ) ../PluginController.class.php:235
5 0.0543 6572432 Git->_dispatchActionAndView( ) ../Git.class.php:439
6 0.0593 7702432 Git_LastPushesGraph->display( ) ../Git.class.php:710
7 0.0626 8926416 Git_LastPushesGraph->displayRepositoryPushesByWeek( ) ../Git_LastPushesGraph.class.php:196