🐛 Disallow empty CTA string in page attachment

[ychsieh]

Also manually add page-attachment script to the template htmls, which is to avoid the script installation errors that only happen in local dev environment.

Context: #39299

[amp-owners-bot]

Hey @gmajoulet, @newmuis, @mszylkowski! These files were changed:

extensions/amp-story-page-attachment/0.1/amp-story-open-page-attachment.js
extensions/amp-story-page-attachment/0.1/test/test-amp-story-open-page-attachment.js

[ychsieh]

cc @Pateriack

[ampprojectbot]

Warning: disparity between this PR Percy build and its main build

The Percy build for this PR was approved (either manually by a member of the AMP team, or automatically if there were no visual diffs). However, during a continuous integration step we generated another Percy build using the commit on the main branch that this PR was merged into, and there appears to be a mismatch between the two.

This is possibly an indication of an issue with this pull request, but could also be the result of flakiness. Please inspect the two builds < This PR's Percy build / main commit's Percy build > and determine further action:

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