Origin with empty string is causing canonical meta not be appended on AMP pages

[jongeyer]

I'm using the amp-module on a project and deploying with Vercel. In this case I know the production url, but I need to also test in preview deployments with dynamically generated base url's. I expect that the origin as empty string ('') should make this work:

...
amp: {
  origin: '',
  mode: 'hybrid'
},
...

Expected result:

• "canonical" link added on AMP pages
• "amphtml" link added on non-AMP pages

Actual result:

• "canonical" link NOT added on AMP pages
• "amphtml" link added on non-AMP pages

The logic for this is in plugin.js:

amp-module/lib/amp/plugin.js

Line 108 in 45dfc3e

if (origin && !head.link.find(l => l.rel === 'canonical' || l.hid === 'canonical')) {

Why is this condition checking if origin is truthy?
The same condition does not apply for appending the amphtml meta.

[jongeyer]

My solution/workaround (specific to vercel):

...
amp: {
  origin: process.env.BASE_URL || `//${process.env.VERCEL_URL}`,
  mode: 'hybrid'
},
...

BASE_URL is localhost for dev environment, and production url on prod environment