Homestyx hydra

R1:bcf15abcd33c

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]…
Repository: R1 hydra
Commit Date: Oct 30 2019