Fix PHP notice when $rule is not a valid array

[jakejackson1]

Description

Return early when $rule is not an array.

Had use report of the following PHP warning showing multiple times in their PDF document:

Warning: Trying to access array offset on value of type null in /wp-content/plugins/gravity-forms-pdf-extended/src/Controller/Controller_Form_Settings.php on line 346

Testing instructions

The only way I could replicate this was by using a filter to modify the rule:

add_filter( 'gform_rule_pre_evaluation', function ( $rule ) {
    return null;
} );

The PR fixes the Gravity PDF-related errors. But if you use the filter, it does cause other errors to be triggered in the PDF by Gravity Forms. It's a case of bad input/bad output, so fixing the input problem (removing the bad filter) is the way forward.

Screenshots

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [ ] 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 76.97%. Comparing base (fd0baa1) to head (e4a7e1e).

:exclamation: Current head e4a7e1e differs from pull request most recent head f99f629. Consider uploading reports for the commit f99f629 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1508 +/- ## ============================================ Coverage 76.97% 76.97% ============================================ Files 244 244 Lines 12740 12741 +1 Branches 370 370 ============================================ + Hits 9807 9808 +1 Misses 2925 2925 Partials 8 8 ```

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