Fix PHP error if WP_Error() object passed to shortcode processor

[jakejackson1]

Description

Customer reported issue with form submission process when using legacy Gravity Forms PayPal add-on. Logs showed $entry was a WP_Error object instead of an array.

Updated code to handle this edge-case.

Testing instructions

Screenshots

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [x] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.94%. Comparing base (761fb81) to head (f625a49).

:exclamation: Current head f625a49 differs from pull request most recent head deb6018. Consider uploading reports for the commit deb6018 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1522 +/- ## ============================================ Coverage 73.94% 73.94% ============================================ Files 191 191 Lines 11453 11453 ============================================ Hits 8469 8469 Misses 2984 2984 ```

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