Gallery Macro in 2.10.x is not able to sort by comments if the images uploaded have combination of comments and without comments.

Steps to reproduce:
1. Upload 2 images and specify comment to the attachments.
2. Upload another few more images without comments to the attachments.
3. Use the following macro:

{gallery:title=Some office photos, and a waterfall|sort=comment}

4. Upon saving/previewing the page, the following exception was thrown:

Error formatting macro: gallery: java.lang.NullPointerException 

Please note that this does not happen in the earlier version of Confluence