Issue #6625: The page to import translation does not import the translation of strings like "<none>"

[avpaderno]

Fixes https://github.com/backdrop/backdrop-issues/issues/6625

[backdrop-ci]

Related to: backdrop/backdrop-issues#6625

[dragonbot]

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[quicksketch]

Merged into 1.x and 1.29.x. Thank you @avpaderno!