[FIX] blockdom: fix event_catcher traceback

[brboi]

When a parent component has an empty child with a t-on and an event is triggered inside the parent, blockdom checks if there is an event_catcher to call. Doing so, an empty child would cause an error.

This commit fixes that.

[brboi]

do not merge for now, it is unclear why we need this fix and what's the cases it covers.

It seems the issue affects only empty child components with a root node and t-foreach directive, but we should dig this.