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]
arcanist(head=master, ref.master=b325304b6e52), phorge(head=master, ref.master=980293b707a0)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/pholio/controller/PholioInlineController.php:117]
```

Closes T15456

Test Plan: After applying this change, try to add an empty Inline Comment in a Pholio mock. See that the error message is now "Comment cannot be empty." instead of "Undefined variable $dictionary".

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15456

Differential Revision: https://we.phorge.it/D25281

src/applications/pholio/controller/PholioInlineController.php

@@ -102,6 +102,7 @@ final class PholioInlineController extends PholioController {
         ->addCancelButton($mock_uri, pht('Close'));
     }
 
+    $error = null;
     if ($request->isFormPost()) {
       $v_content = $request->getStr('content');
 
@@ -112,9 +113,13 @@ final class PholioInlineController extends PholioController {
       } else if ($inline->getID()) {
         $inline->delete();
         $dictionary = array();
+      } else {
+        $error = pht('Comment cannot be empty.');
       }
 
-      return id(new AphrontAjaxResponse())->setContent($dictionary);
+      if ($error === null) {
+        return id(new AphrontAjaxResponse())->setContent($dictionary);
+      }
     }
 
     switch ($mode) {
@@ -151,6 +156,7 @@ final class PholioInlineController extends PholioController {
           ->setUser($viewer)
           ->setName('content')
           ->setLabel(pht('Comment'))
+          ->setError($error)
           ->setValue($v_content));
 
     return $this->newDialog()