Add guard to method

[laritakr]

This is an edge case, but prevents a manifest failure. Previously we got an error because of calling IiifPrint.manifest_metadata_from(work: image, presenter: presenter) when image was nil.

Before & After Screenshots

Before:

After:

Testing Instructions

1. Add a new work with an image file.
2. When viewing the above work, add a child work as a child using the Attach Child dropdown option. Use a PDF file for this child's file.
3. When returning to the original work, universal viewer should show both the original image and all of the child work's split child work images.

[kirkkwang]

Looks good!