Return empty data if fact dimension is missing, not yet available

Summary:
On fresh installation which doesn't have yet any task closed you will not be able to open charts because of error below:
Fixes:
```
[Mon Oct 21 15:42:41 2019] [2019-10-21 15:42:41] EXCEPTION: (TypeError) Argument 1 passed to head_key() must be of the type array, null given, called in ..phabricator/src/applications/fact/chart/PhabricatorFactChartFunction.php on line 86 at [<phutil>/src/utils/utils.php:832]
[Mon Oct 21 15:42:41 2019] #0 phlog(TypeError) called at [<phabricator>/src/aphront/handler/PhabricatorAjaxRequestExceptionHandler.php:27]
[Mon Oct 21 15:42:41 2019] #1 PhabricatorAjaxRequestExceptionHandler::handleRequestThrowable(AphrontRequest, TypeError) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:797]
[Mon Oct 21 15:42:41 2019] #2 AphrontApplicationConfiguration::handleThrowable(TypeError) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:345]
[Mon Oct 21 15:42:41 2019] #3 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:214]
[Mon Oct 21 15:42:41 2019] #4 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:35
```

To fix issue - lets return empty data set instead

Test Plan:
1) Create fresh phabricator installation
2) Create fresh project
3) Try viewing charts

Reviewers: epriestley, Pawka, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, yelirekim

Differential Revision: https://secure.phabricator.com/D20861