3.0: Oembed field type can not be optional

## To Reproduce
Imaginary oembed gallery widget:
```js
module.exports = {
extend: '@apostrophecms/widget-type',
options: {
label: 'Video gallery',
icon: 'video-icon'
},
fields: {
add: {
main: {
type: 'oembed',
name: 'main',
label: 'Main video URL (optional)',
oembedType: 'video'
},
// ...more fields
}
}
};
```
Once something is typed in the `main` field, it can not be deleted, as validation error 'video URL invalid' is preventing the save action of the widget. Additionally, if left empty (it's possible if nothing is typed), the `main` field will contain:
```json
{
"url": null,
"title": "",
"thumbnail": null
}
```
I'm not sure if it's intended, but just `null` sounds better in nunjucks environment because it is counter intuitive and would simplify an eventual template check - `if main` against `if main.url`

## Expected behavior
`oembed` field type should be allowed to be optional.

## Details
Apostrophe version: 3.0.1

Start by reproducing the optional `oembed` field behavior using the widget configuration shown in the issue and Apostrophe 3.0.1. Trace the field's validation and empty-value handling; done means an entered video URL can be cleared, the widget saves when the field is empty, and the empty value has the intended representation.

Written by the indexing model from the issue text.