pmpro_no_quotes - Add null coalescing operator

[ipokkel]

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

When null is passed to the str_replace arguments used in pmpro_no_quotes the follow error got logged in debug:

PHP Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated
Resolves XXX.

Adding a null coalescing operator to the search string third argument returns an empty string which resolves this issue.

How to test the changes in this Pull Request:

1. Activate the Custom Level Cost Text Add On, navigate to Memberships > Settings > Advanced and check the debug log for the deprecated warning.
2. Apply the patch, reload the page, and confirm that no new log entries were added.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

[ideadude]

This works but would raise the PHP version requirement to 7.

Inline with our intention to only raise the version requirement when necessary, we can rewrite this to avoid the null in another way.

Something like if ( empty( $s ) ) { $s = ''; } above the return.