When affixing nav errors, skip empty error blocks

ef3ccf08a707ae1250cdb8d5f2dc6f721d020dc4 introduced a new error block
class. dashboard.js' logic to affix the little red exclamation marks in
the ProductUpdateCreateView then assumed that the forms are full of
errors, even if the error block is empty. Hence empty error blocks are
excluded now.

Also fixed a regression introduced with that commit showing the category
summary.