Optimization: magento/module-tax is_null change to strict comparison

[coderimus]

Hello,
This is the one of the optimization PR linked with the is_null() change. I decided to do them per module just to avoid any mistake during delivery and review.
Best regards,
Alex

Description

Micro-optimizations for Magento\Tax
Magento2 has next sniff MicroOptimizations It checks next: is_null must be avoided. Use strict comparison instead.
Some classes are under // @codingStandardsIgnoreFile such as Magento\Tax module models and cannot be checked during static tests. In this PR I replaced is_null with strict comparison only for models in Magento\Tax module.

Fixed Issues (if relevant)

N/A

Manual testing scenarios

N/A

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)