Allow HTMLSerializer to deserialize an empty doc (#915)

* Allow HTML serializer to deserialize an empty document

This specially handles the case where the HTML is empty.

Test by running:
```
const html = new HTML()
html.deserialize('')
```

Closes #539

* Use defaultBlockType in HTML serializer

* Add test case for deserialising empty string

* Use fixtures

src/serializers/html.js

@@ -98,6 +98,11 @@ class Html {
     const children = $.children().toArray()
     let nodes = this.deserializeElements(children)
 
+    const { defaultBlockType } = this
+    const defaults = typeof defaultBlockType == 'string'
+      ? { type: defaultBlockType }
+      : defaultBlockType
+
     // HACK: ensure for now that all top-level inline are wrapped into a block.
     nodes = nodes.reduce((memo, node, i, original) => {
       if (node.kind == 'block') {
@@ -111,11 +116,6 @@ class Html {
         return memo
       }
 
-      const { defaultBlockType } = this
-      const defaults = typeof defaultBlockType == 'string'
-        ? { type: defaultBlockType }
-        : defaultBlockType
-
       const block = {
         kind: 'block',
         nodes: [node],
@@ -126,6 +126,14 @@ class Html {
       return memo
     }, [])
 
+    if (nodes.length === 0) {
+      nodes = [{
+        kind: 'block',
+        nodes: [],
+        ...defaults
+      }]
+    }
+
     const raw = {
       kind: 'state',
       document: {

test/serializers/fixtures/html/deserialize/empty-string/index.js

@@ -0,0 +1 @@
+export default {}

test/serializers/fixtures/html/deserialize/empty-string/output.yaml

@@ -0,0 +1,7 @@
+data: {}
+nodes:
+  - type: paragraph
+    isVoid: false
+    data: {}
+    nodes:
+      - characters: []