React Native: Patch preview-web and refs to support RN

[dannyhw]

Issue:

What I did

These are some changes discussed previously with @tmeasday @ndelangen and @shilman.

Just hoping I can help move things forward by making this PR.

I applied some small changes in order to enable the v6 of react native storybook. These are:

• making preview and url store optional parameters on the preview web constructor which fixes a crash on react native
• in refs passing a default value when source is not defined in order to bypass a crash in react-native-server

These changes along side this pr on telejson are the main blockers for v6 on react native.

storybookjs/telejson#84

Heres the related react-native-server PR
storybookjs/react-native#393

I'm not sure if there are further implications of these changes and I'm very happy to adjust these changes if they don't make sense.

How to test

• Is this testable with Jest or Chromatic screenshots?
• Does this need a new example in the kitchen sink apps?
• Does this need an update to the documentation?

If your answer is yes to any of these, please make sure to include it in your PR.

[shilman]

These changes look good to me. @tmeasday @ndelangen ?

[ndelangen]

Looks great!

[tmeasday]

WFM!