External update of single value formData to 0 doesn't work.

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https://github.com/RxNT/react-jsonschema-form-conditionals/blob/c834a961d45a09ccbe23bfc30d87fee2a2c22aa9/src/applyRules.js#L80

This should be nextProps.formData !== undefined. Right now it just checks if it is falsey, but 0 is a valid form value.