Fix check for empty multi-name.

Part of #9716.
Fix operation name check in export with a style.

A style name as returned by dt_styles_get_item_list can have a multi-name
catenated. The check to find the corresponding module now take only the
len for the operation.

Fixes #9716.
Merge branch 'fix-export-style'

* fix-export-style:
  Fix operation name check in export with a style.
  Fix check for empty multi-name.