tag_string Field Mandatory

Hi,

I can't seem to make Tag's mandatory to fill in without causing other issues.

In my schema the field tag_string looks like this:
{
"field_name": "tag_string",
"label": "Tags",
"preset": "tag_string_autocomplete",
"required": true
}

The option "required: true" makes the default red * show up next to the field, but does not enforce any type of validation. For example if you leave the field blank, there will be no validation errors.

I see on the [preset code](https://github.com/ckan/ckanext-scheming/blob/master/ckanext/scheming/presets.json#L27) that there is the "ignore_missing" validator, and I tried to replace it for the "not_empty" one.

This at first seemed to be working great, until I try to upload a resource, and when submitting the resource form, I got prompted with the following error:


Any suggestion?

Start with ckanext/scheming/presets.json at the tag_string_autocomplete preset and compare its ignore_missing validator with the attempted not_empty replacement. Reproduce submission of a blank tag_string on the resource form, then verify that required tags produce a useful validation error without causing the reported resource-upload error.

Written by the indexing model from the issue text.