Validate input of comments, without null character

This prevents the creation of a comment with the null character, that
would make the comment invalid for xml output.

Co-authored-by: Victor Pereira <vpereira@suse.com>

src/api/app/models/comment.rb

@@ -7,6 +7,8 @@ class Comment < ApplicationRecord
   validates :body, :commentable, :user, presence: true
   # FIXME: this probably should be MEDIUMTEXT(16MB) instead of text (64KB)
   validates :body, length: { maximum: 65_535 }
+  validates :body, format: { with:    /\A[^\u0000]*\Z/,
+                             message: 'must not contain null characters' }
 
   validate :validate_parent_id