Defaulting to createEmpty if block map is empty in createWithContent (Fixes issue #2226) (#2240)

Summary:
**Summary**

When passing `createWithContent` a `state` parameter created using `convertFromHTML` from an empty string and `createFromBlockArray`, as shown in the example:

```
const sampleMarkup =
  '<b>Bold text</b>, <i>Italic text</i><br/ ><br />' +
  '<a href="http://www.facebook.com">Example link</a><br /><br/ >' +
  '<img src="image.png" height="112" width="200" />';

const blocksFromHTML = convertFromHTML(sampleMarkup);
const state = ContentState.createFromBlockArray(
  blocksFromHTML.contentBlocks,
  blocksFromHTML.entityMap,
);

this.state = {
  editorState: EditorState.createWithContent(
    state,
    decorator,
  ),
};
```

The following error is thrown:

```
TypeError: Cannot read property 'getKey' of undefined

  84 | EditorState.createWithContent = function createWithContent(contentState, decorator) {
> 85 |   var firstKey = contentState.getBlockMap().first().getKey();
     | ^  86 |   return EditorState.create({
  87 |     currentContent: contentState,
  88 |     undoStack: Stack(),
```
The previous error is generated because `createWithContent` asumes that `contentState.getBlockMap().first()` will return an element, which is wrong for scenarios where the block map is empty. As a consecuence the `getKey();` function in `contentState.getBlockMap().first().getKey();` is executed on `undefined`, thus throwing the error.

The previous scenario is very common if you are planning to use the editor to allow a user to write HTML content which could also be blank, especially as a default value.

**Solution**

In order to make the less amount of modifications to the code, I've added a validation that will run `createEmpty` in case the block map is empty.

**Test Plan**

Open `/draft-js/examples/draft-0-10-0/convertFromHTML/convert.html` and change line #61 from:

`const blocksFromHTML = convertFromHTML(sampleMarkup);`

to:

`const blocksFromHTML = convertFromHTML('');`

No error will be shown.
Pull Request resolved: #2240

Differential Revision: D18247644

Pulled By: mrkev

fbshipit-source-id: 3eb90fd5379e8a2d85efbb2b7b9587ca87701d12

Author: davoscript

meta/bundle-size-stats/Draft.js.json

@@ -75,6 +75,9 @@ class EditorState {
     contentState: ContentState,
     decorator?: ?DraftDecoratorType,
   ): EditorState {
+    if (contentState.getBlockMap().count() === 0) {
+      return EditorState.createEmpty(decorator);
+    }
     const firstKey = contentState
       .getBlockMap()
       .first()