[FIX] ir.translation: avoid duplicating translations

Rev. 253437f8 meant to
avoid the creation of duplicate entries during translation
import, caused by existing translations created with an
empty `module` field.

But the fix was over-zealous and caused an extra side-effect:
the creation of missing translations (the blue flag icon)
now creates duplicate translations if other translations
exist with different `module` values.
(This could happen because translations are provided by
several modules, or due to manual translation entries with
no module value, etc.)