[importer] accept "No publication date" as valid entiry in source:publication_timestamp

[tlongers]

Values in source:publication_timestamp are validated against the ISO standard:

https://github.com/security-force-monitor/sfm-cms/blob/c8a00b435ac57732d767617c28eef264aed9f9d4/sfm_pc/management/commands/import_country_data.py#L1455C3-L1470

We have settled on "No publication date" as a human comprensible explicit NULL value we use to assert that we know a source doesn't have publication date (as opposed to the case where we've not included one in error). However, the importer throws an error when this string is present in the attribute.

For now, I think it is practical to update the importer to accept "No publication date" as a valid entry in this field. The open question is whether this value is stored, or an empty value is recorded in the database.

[hancush]

Also makes sense.