Add default value for undefined case in markdown transformers

[imnoahcook]

Easy review

Context:
I was playing with the rich text playground and noticed that if you only used the ELEMENT_TRANFORMERS from @lexical/markdown, you will get a runtime error as it tries to iterate over the textFormat and textMatch fields, which are undefined in this case.

They're undefined because none of the element transformers have these fields in them.

I consider this a bug, because there shouldn't be any reason someone not to be able to use

for quote

1. for list
   etc..
   without using the rest of the markdown exports

Solution:
Add a default empty array value to text-format and text-match

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The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
lexical	✅ Ready (Inspect)	Visit Preview	Jul 7, 2022 at 4:35AM (UTC)
lexical-playground	✅ Ready (Inspect)	Visit Preview	Jul 7, 2022 at 4:35AM (UTC)

[imnoahcook]

Screenshot for context on what happens when you try to add `ELEMENT_TRANSFORMERS` to the `MarkdownShortcutPlugin` on the `rich-text-playground` example

[acywatson]

Just needs the CLA signed, when you have a chance.

[imnoahcook]

Signed CLA ✔️

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[acywatson]

Ah, I forgot, we don't use nullish coalescing in Lexical for some reason that I can't quite recall at the moment. Something. about not wanting to set up the Babel plugin to transpile it, I think. You can use a logical OR, instead.

[fantactuka]

Minor update, looks good otherwise!

[acywatson]

It looks like the playground vercel build is failing because of a missing script that was added recently. Rebasing might fix that.

[imnoahcook]

Resolved changed by @fantactuka and rebased to pass tests.