Require valid comments to contain at least one non-whitespace character

Summary:
See downstream <https://phabricator.wikimedia.org/T88655>. This is very marginal, but we currently allow comments consisting of //only// whitespace.

These are probably always mistakes, so treat them like completely empty comments.

(We intentionally do not trim leading or trailing whitespace from comments when posting them becuase leading spaces can be used to trigger codeblock formatting.)

Test Plan:
  - Posted empty, nonempty, and whitespace-only comments.
  - Whitespace-only comments now have the same behavior as truly empty comments (e.g., do not actually generate a transaction).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20562

src/applications/transactions/storage/PhabricatorApplicationTransaction.php

@@ -127,7 +127,19 @@ abstract class PhabricatorApplicationTransaction
   }
 
   public function hasComment() {
-    return $this->getComment() && strlen($this->getComment()->getContent());
+    if (!$this->getComment()) {
+      return false;
+    }
+
+    $content = $this->getComment()->getContent();
+
+    // If the content is empty or consists of only whitespace, don't count
+    // this as comment.
+    if (!strlen(trim($content))) {
+      return false;
+    }
+
+    return true;
   }
 
   public function getComment() {