Core: Fix useParameter with nullish coalescing

[evancharlton]

Issue

The useParameter(..) hook accepts a key and a default value. It then
provides the default value if the current story has not provided a
desired value for the relevant parameters object.

What I did

This change updates the logic of when to fall back to the default. The
old logic used a boolean operator (||), which worked as long as the
provided value wasn't falsey. Instead, account for this by using the
nullish coalescing operator (??).

How to test

1. Create an addon which consumes a parameter like:

   const parameters = useParameter("myAddon", { defaultValue: true });

2. Create a story which looks like:

   export const Bug = {
     parameters: {
       myAddon: false
     }
   };

3. Note that the value of parameters in your addon will always be
   the default object, not false (as was requested by the story).

• 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.

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[yannbf]

Hey @evancharlton thank you for your contribution! Looks great 👍