Empty strings are incorrectly allowed as example string prop values in JavaScript component config schema

Sibling of #3586958: Relative image paths are incorrectly allowed as example image prop values in SDCs and code component example values.

I thought this would end up in the same place as #3586958: Relative image paths are incorrectly allowed as example image prop values in SDCs and code component example values, but it's not quite so can be committed separately.

I fail to remember why an empty example is not valid iff the prop is not required.

#7: Because:

1. Conceptually: how is an empty string an actual example value? Why even provide that as an example?
2. Field types will actually filter away empty values, see \Drupal\Core\Field\FieldItemListInterface::filterEmptyItems() + \Drupal\Core\TypedData\ComplexDataInterface::isEmpty(). For example \Drupal\Core\Field\Plugin\Field\FieldType\UriItem::isEmpty() does:
   

     public function isEmpty() {
       $value = $this->getValue();
       if (!isset($value['value']) || $value['value'] === '') {
         return TRUE;
       }
       return parent::isEmpty();
     }
   

   … which is directly leading to the exception this issue reported.

#7: I suspect you're thinking of "empty string is not a valid string enum value" which you worked on.

          // The empty string is not a sensible enum value. To indicate
          // optionality, the prop should be made optional.

I definitely had #9 in mind, I failed to see why I wouldn't accept an empty string for a regular string.
So in this case is a core limitation.

Re: Conceptually: I disagree. An empty string would be a good example for reenforcing that it's an optional prop. The problem is that we are using the first one. Wouldn't be an issue if we took the first-not-empty-one as default or if #3514672: [Needs design] DX & authoring experience: support `default` in addition to `examples[0]`, enables updating a component input across all existing instances was completed.

+1 to RTBC btw. #11 is out of scope here.

#11:

An empty string would be a good example for reenforcing that it's an optional prop.

You're looking at it from a Canvas end user/Content Author POV. You should look at it here from a component developer POV, because that's the person who is now going to be told to do something differently.

When a SDC or code component prop is marked as optional, it's optional from a JSON Schema POV. Which means "it can be omitted". The absence of a value is not the same as a value considered empty. Which value would you consider "empty" for type: integer? And which one for type: string, format: uri?

Therefore this is IMHO a slippery slope that will lead to more confusion. I disagree with allowing '' as a valid example value.

IOW: this is about code/metadata hygiene. And Canvas' ability to reason about the JSON schema for a given component. This MR has zero impact on end users.

The absence of a value is not the same as a value considered empty.

True that.