Allow loading prop on Rehydrated to accept null values

[3nvi]

Issue #, if available:
#422

Description of changes:
This PR simply checks loading against an undefined value, instead of a falsy one. The PR also updated the README of the subrepo in order to document the props that the Rehydrated component accepts

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[matthamil]

I'm not sure there's much of a need for the loading prop to accept a null value. If you don't want to render anything with the <Rehydrated /> component, you can use the render prop to provide a function that returns null, which is an existing component API.

Imo, a nicer win would be to remove the "Loading..." message that appears by default when the rehydration is in progress. If the <Rehydrated /> component wraps an entire screen (or a large portion of one), it's quite jarring to see a white screen with "Loading..." for a moment prior to switching to the screen it was meant to render.