Which code are you talking about?
I have AbstractDocumentBuilder.bindSpatialAnnotation being called to initialize class level spatial constraints.

Ok, we're talking about the same method.
But this is what I see there, with ann being the instance of @Spatial :

String fieldName;
if ( !ann.name().isEmpty() ) {
    fieldName = prefix + ann.name();
}
else {
    fieldName = clazz.getName(); <--- The case we're talking about, right?
}

I don't understand which exception is thrown; if you look at example domain entity: org.hibernate.search.test.spatial.UserEx that's valid and works fine without exceptions even while one of the Spatial annotations doesn't define a value for the name() attribute.