Not with the way the converter is currently set up. The `self.validate` function a [few lines up](https://github.com/reciprocity/ggrc-core/blob/1b09cfe0157da241ddd20a6d1f53d0134eb5caa7/src/ggrc/converters/base_row.py#L245) only checks for whether the item is required and absent so as to trigger an error.

Validation of input when it is given at all is actually handled by the `parse_item` function.  The handler for the assignee column is treated as a special case.

So we may want to rename `validate()` to something like `enforce_required()` to more faithfully reflect what it does, but it's working as intended.