Add null return types to BlockRenderer and defaultBlockTag

[pereznieto]

Description

This PR adds two null types to the TypeScript definition of draft-js-export-html: BlockRenderer and defaultBlockTag.
These null return types are present in the documentation (and code), but were missing from the TypeScript definition, thus causing type linting/compilation errors.
I decided to combine both changes in a single PR, as the changes are small and simple enough and touch the same file.

Changes

BlockRenderer

The docs state:

You can return a string to render this block yourself, or return nothing (null or undefined) to defer to the default renderer.

The possibility to return null or undefined was missing from the TypeScript definition, meaning it wasn't possible to defer to the default renderer in a BlockRenderer function.

defaultBlockTag

The docs state:

If you don't want any parent block tag, you can set defaultBlockTag to null.

The TypeScript definition didn't allow for this, as it required a string to be returned.

Related issues

• BlockRenderer: BlockRenderer type can be null #132
• defaultBlockTag: defaultBlockTag type can accept null  #197

[shpyo]

Suggested change

	type BlockRenderer = (block: draftjs.ContentBlock) => string|null|undefined;
	type BlockRenderer = (block: draftjs.ContentBlock) => ?string;