Issue-864: Resolve PHP Deprecations

[anubisthejackle]

Description

While running the unit tests, I run into several PHP deprecations. It'd be great if those issues were resolved since they create noice in logs.

PHP Deprecated:  strlen(): Passing null to parameter #1 ($string) of type string is deprecated in /var/www/wordpress/wp-content/plugins/wordpress-fieldmanager/php/class-fieldmanager-group.php on line 406

PHP Deprecated:  strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /var/www/wordpress-develop/src/wp-includes/functions.php on line 7127

PHP Deprecated:  strlen(): Passing null to parameter #1 ($string) of type string is deprecated in /var/www/wordpress/wp-content/plugins/wordpress-fieldmanager/php/class-fieldmanager-field.php on line 992

... more ...

Also, consider using convertDeprecationsToExceptions="true" in the phpunit.xml so that a test fails if it contains PHP deprecations.

Use Case

Testing the test suite returns no PHP deprecations.

Acceptance Criteria

• Add checks to ensure that values are strings before running them through string functions in the three locations identified above
• Update the unit test configuration so that deprecations are converted to exceptions and require resolution before merging a PR

[kevinfodness]

A few things to address before merge, otherwise 🍣