Handle case in Site Scanning when no scannable URL is available

[delawski]

Summary

Fixes #6855

In the Reader mode, only post types are used for performing a site scan. If there are no posts, pages or other custom post types available for a scan, a request made to the /wp-json/amp/v1/scannable-urls endpoint returns an empty array.

With this change, an empty array is treated as a valid response so that the list of scannable URLs kept in the Site Scan state is emptied. This case is then handled gracefully on the frontend where an error message is rendered. The message says that there are no URLs available for a scan.

Even though the error message has already been part of the Site Scan UI, it was displayed only if there were no scannable URLs available initially, right after a page load.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

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[codecov]

Codecov Report

Merging #6856 (3e03e12) into develop (0bb7948) will decrease coverage by 0.81%.
The diff coverage is 100.00%.

❗ Current head 3e03e12 differs from pull request most recent head 1dec195. Consider uploading reports for the commit 1dec195 to get more accurate results

@@              Coverage Diff              @@
##             develop    #6856      +/-   ##
=============================================
- Coverage      78.36%   77.55%   -0.82%     
  Complexity      6721     6721              
=============================================
  Files            202      267      +65     
  Lines          20276    21460    +1184     
=============================================
+ Hits           15890    16644     +754     
- Misses          4386     4816     +430     

Flag	Coverage Δ
javascript	63.68% <100.00%> (?)
php	78.36% <ø> (ø)
unit	78.36% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...src/components/site-scan-context-provider/index.js	34.48% <100.00%> (ø)
assets/src/block-editor/store/selectors.js	100.00% <0.00%> (ø)
...onents/site-scan-results/site-scan-sources-list.js	100.00% <0.00%> (ø)
assets/src/components/amp-setting-toggle/index.js	100.00% <0.00%> (ø)
assets/src/common/helpers/is-external-url.js	100.00% <0.00%> (ø)
...block-validation/components/error/error-content.js	100.00% <0.00%> (ø)
...sets/src/block-validation/components/icon/index.js	100.00% <0.00%> (ø)
assets/src/components/amp-info/index.js	100.00% <0.00%> (ø)
...s/src/components/options-context-provider/index.js	2.17% <0.00%> (ø)
...idation/components/error/get-error-source-title.js	100.00% <0.00%> (ø)
... and 56 more

[westonruter]

Behavior checks out. I registered a Recipe custom post type that had no posts created for it.

Before

Before once I updated the supported post types to only have Recipes, the Site Scan allowed by to initiate a scan but then it failed:

before.mov

After

After the fix, as soon as I saved the settings it immediately showed the appropriate error message up front:

after.mov

[yogeshbeniwal]

@westonruter For the scenario where few of the post type have post and few doesn't. What will happen in that case. Will site scan continue or throws error.

[westonruter]

@yogeshbeniwal As long as one of the post types have an AMP URL available, the site scan should proceed as expected.

@fellyph Would you confirm?

[fellyph]

@yogeshbeniwal @westonruter

Yes, only the post type with one or more posts will be part of the scan. This fix was for users who select a post type with no posts.

[yogeshbeniwal]

Thank you @westonruter @fellyph for confirmation.