[FIX] orm: do not save empty translation as 'false'

When writting a value on a translatable field in a different language than
English, the submitted *raw* value was saved in the database.

This could cause the following issues:
- empty value (provided as `False` by the web client) saved as the string
  'false' in the translations table
- no encoding or sanitization convertion
- ignore size parameter on the translatable field

Process the submitted translation through symbol_set method to clean it before
storing it blindly in the database.

This allows to convert `False` into `''` for empty value and fixes #10862