AMP validation error due to to empty LD+JSON script

[westonruter]

Bug Description

When testing in AMP plugin v1.5.0-RC1, I'm getting a validation error with code JSON_ERROR_EMPTY for a script emitted by Site Kit:

<script type="application/ld+json" id="__gaOptOutExtension"></script>

See google/site-kit-wp#1251 and introduced in google/site-kit-wp#1252.

Nevertheless, that HTML is not invalid AMP:

It appears the JSON_ERROR_EMPTY is not applicable to script elements with of type application/ld+json. The validator spec for LD+JSON is:

# JSON Linked Data
tags: {
  html_format: AMP
  html_format: AMP4ADS
  html_format: AMP4EMAIL
  html_format: ACTIONS
  tag_name: "SCRIPT"
  spec_name: "script type=application/ld+json"
  attr_lists: "nonce-attr"
  attrs: {
    name: "type"
    mandatory: true
    value_casei: "application/ld+json"
    dispatch_key: NAME_VALUE_DISPATCH
  }
  cdata: {
    blacklisted_cdata_regex: {
      regex: "<!--"
      error_message: "html comments"
    }
  }
}

No error is omitted for <amp-ima-video> either:

Its spec is:

# AMP IMA VIDEO
tags: {
  html_format: AMP
  html_format: ACTIONS
  tag_name: "SCRIPT"
  spec_name: "amp-ima-video > script[type=application/json]"
  mandatory_parent: "AMP-IMA-VIDEO"
  attrs: {
    name: "type"
    mandatory: true
    value_casei: "application/json"
    dispatch_key: NAME_VALUE_PARENT_DISPATCH
  }
  cdata: {
    blacklisted_cdata_regex: {
      regex: "<!--"
      error_message: "html comments"
    }
  }
}

Here it has the same CDATA spec, but one emits an warning and the other emits nothing.

It's the same for amp-analytics:

So the quick fix appears to be to eliminate the JSON_ERROR_EMPTY error code. It was introduced in #4340 (for #4320). The validator emits this as a warning:

            this.context_.addWarning(
                amp.validator.ValidationError.Code.INVALID_JSON_CDATA,
                this.context_.getLineCol(), /* params */[], '',
                this.validationResult_);

So since addWarning is being used as opposed to addError, the JSON_ERROR_EMPTY error should be removed. It turns out that application/ld+json scripts are not validated at all.

Steps to reproduce

1. Enable AMP plugin with Standard or Transitional mode.
2. Activate Site Kit with analytics.
3. Validate an AMP page.
4. See validation error.

Screenshots

Additional Context

• Plugin Version: 1.5.1

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