R1:7c1487e4dcfa

Fix Pholio RuntimeException: Undefined variable $dictionary (when adding an empty Inline Comment)

Summary:
Trying to create an empty inline comment in a Pholio mock, `$dictionary` does not get set as both `strlen($v_content)` and `$inline->getID()` are not true.

Thus show a more explanatory error message ('Comment cannot be empty.') to users instead of exposing internal variable names.

```
EXCEPTION: (RuntimeException) Undefined variable $dictionary at [<arcanist>/src/error/PhutilErrorHandler.php:261]…