[FIX] web: use rec_name to display imported record

When expending a many2x record, a field named 'name' was used to
represent the exported record.
If there was no field 'name' on the record, expanding the many2x field
raised a KeyError in the export screen with "import-compatible export"
enabled.

Use the _rec_name instead as fallback

closes odoo/odoo#40768

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>