Fix array_key_exists on objects (deprecated notice or fatal error)

[luigifab]

Description

This fix a notice or a fatal error when creating a new product in backend, I hope it's good.

PHP 7.4.6
array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead

PHP 8.0.0-rc2
Fatal error: Uncaught TypeError: array_key_exists(): Argument #2 ($array) must be of type array, Mage_Core_Model_Config_Element given

OpenMage 20.0.5 / PHP 7.4.6 and 8.0.1

Manual testing scenarios

1. Install PHP 7.4 or 8.0
2. Go to, Catalog → Manage products
3. Create a new product
4. Press continue button

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (all builds are green)

[joshua-bn]

Any reason to not just use isset() there?

[luigifab]

No special reason, I have choose one of the solution.

[github-actions]

Unit Test Results

1 files  ±0  1 suites  ±0   0s ⏱️ ±0s
0 tests ±0  0 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
2 runs  ±0  2 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit a04c1b2. ± Comparison against base commit 5ddb7a3.