Hot Fix: Prevent fatal error on save posts or pages with [give_multi_form_goal] shortcode

[glaubersilva]

Resolves #7060

Description

We are getting a fatal error when users try to use the multi-form shortcode with the ids attribute.

Something like this: [give_multi_form_goal ids="31,39]

This PR fixes this bug - by returning the proper data type in the getForms() method - and allows users to use the ids attribute without throwing any fatal error.

Affects

The give_multi_form_goal shortcode.

Testing Instructions

1. Add the Multi-Form Goal Shortcode to a page or post and include the "ids" parameter to only include one or more forms. Example: [give_multi_form_goal ids="31,39]
2. Click on "Update"
3. No fatal errors should be thrown and the form should get updated without problems.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed

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[JoshuaHungDinh]

G2G in my tests.

[rickalday]

Passed manual QA tests