renderToJson return null when a component returns null

[pedrottimark]

• Return an explicit null value instead of implicit undefined value so that renderToJson is consistent with shallowToJson
• Added test for renderToJson (and also for mountToJson to assert its behavior) compare to https://github.com/adriantoine/enzyme-to-json/blob/master/tests/core/shallow.test.js#L36-L41

[codecov-io]

Current coverage is 100% (diff: 100%)

Merging #41 into master will not change coverage

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[adriantoine]

Hi! That looks good, thanks for your contribution! 👌