Fix Calls to count()

[bbatsche]

Description

Add logic and checks to avoid passing null to count() and make sure that any classes with a public count() method implement Countable interface.

Fixed Issues (if relevant)

1. magento-engcom/php-7.2-support#7: Warning: count(): Parameter must be an array or an object that implements Countable

Contribution checklist

• [ ] Pull request has a meaningful description of its purpose
• [ ] All commits are accompanied by meaningful commit messages
• [ ] All new or changed code is covered with unit/integration tests (if applicable)
• [ ] All automated tests passed successfully (all builds on Travis CI are green)

[buskamuza]

There are still failures with count(). See:

• https://travis-ci.org/magento-engcom/php-7.2-support/jobs/355658287
• https://travis-ci.org/magento-engcom/php-7.2-support/jobs/355658288
• https://travis-ci.org/magento-engcom/php-7.2-support/jobs/355658291
• https://travis-ci.org/magento-engcom/php-7.2-support/jobs/355658292
• https://travis-ci.org/magento-engcom/php-7.2-support/jobs/355658293