Avoid PHP warnings by adding null defaults

[bor0]

Context

• Use similar approach in check_limit() as we do in get_upgrade_campaign().

Summary

This PR can be summarized in the following changelog entry:

• Adds safety check when accessing an array to avoid PHP warnings when getting the upgrade campaign for Wincher integration. Props to @bor0.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Go to the Wordpress dashboard ad click on the wincher widget.
• Connect to wincher account or apply this patch to trigger the upgrade check
  wincher-upgrade.patch
• Add to the wp-config.php the following lines:

define( 'WP_DEBUG', true ); 
define( 'WP_DEBUG_LOG', true ); 
define( 'WP_DEBUG_DISPLAY', false );

• Go back to the Wordpress dashboard and refresh.
• Open the debug.log from the wp-content folder and search for Warning: Undefined array key "months"
• Check you don't see the warning.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched of
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #

[vraja-pro]

CR & AC ✅