Add back support for NULL values to Cartesian Argument Providers

[petrandreev]

During migration to JDK11 the support of NULL values provided by CartesianParameterArgumentsProvider has been silently dropped.

These changes

• add back support for NULL values and ensure backward compatibility with v. 2.0.1
• add regression tests for provided NULL values support

Fixes: #764

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[Michael1993]

LGTM.

I did not know about this difference between toList and toUnmodifiableList, thank you! Would you like to add yourself to the README as a contributor? 🙂

[petrandreev]

Thanks, added

[Michael1993]

@petrandreev Hi!

This PR is already merged, thank you for contributing it! Do you have some thoughts or something specific you'd like to discuss or address? I'm here to help with any concerns you might have.