PackageComment: default RenderedComment to str()

With this, `bool(PackageComment.RenderedComment) == False`

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/models/package_comment.py

@@ -66,7 +66,4 @@ class PackageComment(Base):
                 params=("NULL"))
 
         if self.RenderedComment is None:
-            raise IntegrityError(
-                statement="Column RenderedComment cannot be null.",
-                orig="PackageComments.RenderedComment",
-                params=("NULL"))
+            self.RenderedComment = str()