The third-party list template emitted a text node for every optional
field even when the value was empty, producing `{"text":""}` nodes that
violate the ProseMirror schema and make Tiptap refuse to render the
document with "Empty text nodes are not allowed".
Add a `default` template helper and substitute "—" for empty values in
third_party_list.json.tmpl, and add a migration that rewrites existing
document_versions.content to drop any empty text nodes (per-row safe,
preserves marks/attrs/ordering, leaves updated_at untouched).
Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>
797e3da52f