[IMP] translate: export empty translations for empty ones

Instead of use the original string by default.

Currently odoo export a PO translation file using the logic:
"If there is not a translation then use the source."

This generate a false 100% of file translated for tools based on PO
files.

This commit change this logic to:
"If there is not a translation then use empty string."

Odoo import a PO translation file supporting empty string because
if a item is empty string odoo use the original source.

Closes #17925

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