Allow image name to be empty for simple list test

An empty name is valid for an image, but the list_images scheme
is missing the null option, so test_list_images test fails.

This commit brings the list_images scheme in line with the
list_images_detailed schema and allows the name field to be empty,
and both test_list_images and test_list_images_with_detail tests
pass.

Change-Id: I40f05fc02c199747065830f7a7719680bbaa8d71
1 file changed