output_validators not effecting package_extras not in database

### CKAN Version
2.10.8

### Describe the Bug
When an extra in the `package_extra` database table is not populated/does not exist, `output_validator`s won't run on that extra field.

### Replicating the bug

**Input**:
Call `/api/3/action/package_show?id=id123` with the following body

- package `id123` doesnt have a value for "collection_method" in the package_extra database table
- package `id123` is of `dataset_type: "dataset"`

**Expected Output**:
- "collection_method" field is shown as collection_method: null

**Actual Output**:
- "collection_method" field is not shown in package_show response

### Configurations

I am using this YAML to configure scheming:

```
scheming_version: 2
dataset_type: dataset
about: A reimplementation of the default CKAN dataset schema
about_url: http://github.com/ckan/ckanext-scheming

dataset_fields:
- field_name: title
label: Title
required: true

- field_name: url
label: URL
preset: dataset_slug
form_placeholder: eg. my-dataset
required: true

- field_name: collection_method
label: Collection Method
output_validators: default_to_none

resource_fields:
- field_name: title
label: Title
required: true

- field_name: url
label: URL
preset: dataset_slug
form_placeholder: eg. my-dataset
required: true

```

It references this custom validator:

```
def default_to_none(value):
if value:
return value
else:
return None
```

I have been refreshing the SOLR index before and after making the `package_show` call.