Fix #34689: Multilayer EXRs don't load correctly if one layer is named and the other is not

OpenEXR's ChannelList.layers() will not include empty layer names,
which caused the issue. Made it so if there's one non-empty layer
name, check for empty names happens.

If there's empty layer name in case one non-empty layer name, file
will be considered multilayer now.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55498 954f8c5b-7b00-dc11-b283-0030488c597c