BlockRenderer type can be null

[nilaratna]

I have a case where I need to conditionally skip the custom render when using a custom block renderer map. The code already supports the ability to skip if null is returned but this is blocked by type linting. Can we make changes to the TypeScript and Flow definitions to allow BlockRenderer to be string or null. This is my sample code:

blockRenderers: {
    unstyled: (block: Draft.ContentBlock): string | null => {
      if (block.getText().trim() === '') {
        return '';
      }
      return null;
    }
  }

I can open a PR to do this but wanted to see if there were any arguments against this first.

The change would be to

draft-js-utils/packages/draft-js-export-html/typings/index.d.ts

Line 8 in 7174891

type BlockRenderer = (block: draftjs.ContentBlock) => string;

and

draft-js-utils/packages/draft-js-export-html/src/stateToHTML.js

Line 33 in 0ff3a0c

type BlockRenderer = (block: ContentBlock) => ?string;

[pereznieto]

My team was also being affected by this issue, so I've raised a PR to add the suggested change: #201