[FIX] core: fix translation dialog

The fix in #106174 was not good.
The translation dialog cannot update not translated terms

the initial term value may be empty "" when the source and the translation are
the same at first. So, the source to for update_field_translations should be
  const source = term.value ? term.value : term.source;

closes odoo/odoo#106360

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>