Calendar search form allows users to define date ranges. Entering gibberish data leads to a cryptic exception due to calling `format()` on `null`, as `AphrontFormDateControlValue::getDateTime()` can return `null` instead of a `DateTime` object.
Also add some additional PhpDoc as a result of playing with this code.
Note that other calendar query forms are more lenient and still accepts gibberish after applying this patch. The intention behind this patch is replacing a cryptic exception with a more appropriate and descriptive error; this patch does not attempt to introduce validation everywhere.
```
EXCEPTION: (Error) Call to a member function format() on null at [<phorge>/src/applications/calendar/query/PhabricatorCalendarEventSearchEngine.php:469]
```
Closes T15943