Fix null value in ResultPath

[tuliocasagrande]

Issue #, if available:
#45

Description of changes:
This PR fixes the output null value in ResultPath to behave as described here: https://docs.aws.amazon.com/step-functions/latest/dg/input-output-resultpath.html#input-output-resultpath-null

The argument result_path=None in Python is going to be converted to "ResultPath": null in JSON.

I tested with Task, but it could be tested with any state type as I believe "ResultPath": null is a behavior available for any type.

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[shunjd]

Hi @tuliocasagrande, thank you for the fix! Could you also add InputPath and OutpuPath too?

[tuliocasagrande]

Sure, no problem.
I didn't know "InputPath": null and "OutpuPath": null were also valid options.

[tuliocasagrande]

@shunjd I pushed a new commit with InputPath and OutputPath. Let me know if the tests are OK or if it's better to make one test for each parameter.

Thanks again!

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[vaib-amz]

Thanks for these fixes!