Shortcode: Fix issue when no attributes are passed

[AnuragVasanwala]

Context

This PR fixes #13262 by adding initialization check for the attributes as described in the issue.

Summary

When no argument is passed to the shortcode raises following error:

Fatal error: Uncaught TypeError: Google\Web_Stories\Shortcode\Stories_Shortcode::render_stories(): Argument #1 ($attrs) must be of type array, string given, called in /wp-includes/shortcodes.php on line 355 and defined in /wp-content/plugins/web-stories/includes/Shortcode/Stories_Shortcode.php on line 67

As per the issue #13262, I have addressed changes mentioned and tested it.

TODO

• Add unit-testing for the PR

Testing Instructions

1. Insert shortcode block in post editor
2. Insert [web_stories /]
3. Try to save post
4. Get errors

• This is a non-user-facing change and requires no QA

Reviews

Does this PR have a security-related impact?

No.

Does this PR change what data or activity we track or use?

No.

Does this PR have a legal-related impact?

No.

Checklist

• This PR addresses an existing issue and I have linked this PR to it in ZenHub
• I have tested this code to the best of my abilities
• This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• I have added documentation where necessary

---

Fixes #13262

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[swissspidy]

Thanks a lot!