fix bugs due to Slate "nodes" properties being boolean

src/components/Widgets/Markdown/MarkdownControl/VisualEditor/__tests__/__snapshots__/parser.spec.js.snap

@@ -241,7 +241,6 @@ Object {
     Object {
       "isVoid": true,
       "kind": "block",
-      "nodes": undefined,
       "type": "thematic-break",
     },
     Object {
@@ -286,7 +285,6 @@ Object {
     Object {
       "isVoid": true,
       "kind": "block",
-      "nodes": undefined,
       "type": "thematic-break",
     },
     Object {
@@ -1200,7 +1198,6 @@ more important, there there are only so many sizes that you can use.",
     Object {
       "isVoid": true,
       "kind": "block",
-      "nodes": undefined,
       "type": "thematic-break",
     },
     Object {

src/components/Widgets/Markdown/serializers/remarkSlate.js

@@ -58,6 +58,14 @@ const markMap = {
 };
 
 
+/**
+ * Add nodes to a parent node only if `nodes` is truthy.
+ */
+function addNodes(parent, nodes) {
+  return nodes ? { ...parent, nodes } : parent;
+}
+
+
 /**
  * Create a Slate Inline node.
  */
@@ -67,7 +75,8 @@ function createBlock(type, nodes, props = {}) {
     nodes = undefined;
   }
 
-  return { kind: 'block', type, nodes, ...props };
+  const node = { kind: 'block', type, ...props };
+  return addNodes(node, nodes);
 }
 
 
@@ -75,7 +84,8 @@ function createBlock(type, nodes, props = {}) {
  * Create a Slate Block node.
  */
 function createInline(type, props = {}, nodes) {
-  return { kind: 'inline', type, nodes, ...props };
+  const node = { kind: 'inline', type, ...props };
+  return addNodes(node, nodes);
 }
 
 
@@ -143,7 +153,7 @@ function processMarkNode(node, parentMarks = []) {
 function convertMarkNode(node) {
   const slateNodes = processMarkNode(node);
 
-  const convertedSlateNodes = slateNodes.reduce((acc, node, idx, nodes) => {
+  const convertedSlateNodes = slateNodes.reduce((acc, node) => {
     const lastConvertedNode = last(acc);
     if (node.text && lastConvertedNode && lastConvertedNode.ranges) {
       lastConvertedNode.ranges.push(node);

src/components/Widgets/Markdown/serializers/remarkSquashReferences.js

@@ -55,7 +55,8 @@ export default function remarkSquashReferences() {
 
       const pre = u('text', node.type === 'imageReference' ? '![' : '[');
       const post = u('text', ']');
-      return [ pre, ...children, post];
+      const nodes = children || [ u('text', node.alt) ];
+      return [ pre, ...nodes, post];
     }
 
     /**