[FIX] ir_qweb: regression of 8395f0d2

This revision corrects a regression introduced by
8395f0d2:
the possibility to call
`ir.qweb.field.datetime`.`value_to_html` with
a datetime string directly, instead of calling
`ir.qweb.field.datetime`.`record_to_html`
(which call `value_to_html` as well).

opw-675427