Fix empty alt value for imageReference, image

In the below example, the image node had an `alt` property set to `null`,
whereas the reference had an `alt` set to `""`.

```md
![](/xyz.png)

![][1]

[1]: /xyz.png
```

This update ensures both have `null` as the value corresponding to an
empty `alt`.

Closes GH-103.

lib/parse.js

@@ -4577,10 +4577,6 @@ function tokenizeReference(eat, value, silent) {
  index++;
  }
 
- if (!queue) {
- return;
- }
-
  subvalue = text = queue;
  character = value.charAt(index);
 
@@ -4606,6 +4602,10 @@ function tokenizeReference(eat, value, silent) {
  character = value.charAt(index);
 
  if (character !== C_BRACKET_OPEN) {
+ if (!text) {
+ return;
+ }
+
  identifier = text;
  } else {
  identifier = EMPTY;
@@ -4699,7 +4699,7 @@ function tokenizeReference(eat, value, silent) {
  node.children = self.tokenizeInline(text, now);
  exitLink();
  } else if (type === T_IMAGE) {
- node.alt = decode(self.descape(text), eat);
+ node.alt = decode(self.descape(text), eat) || null;
  }
 
  return eat(subvalue)(node);

lib/stringify.js

@@ -1954,7 +1954,7 @@ compilerPrototype.linkReference = function (node) {
  * @return {string} - Markdown image reference.
  */
 compilerPrototype.imageReference = function (node) {
- var alt = this.encode(node.alt, node);
+ var alt = this.encode(node.alt, node) || EMPTY;
 
  return EXCLAMATION_MARK +
  SQUARE_BRACKET_OPEN + alt + SQUARE_BRACKET_CLOSE +

test/input/reference-image-empty-alt.text

@@ -0,0 +1,3 @@
+![][1]
+
+[1]: /xyz.png

test/tree/reference-image-empty-alt.json

@@ -0,0 +1,65 @@
+{
+ "type": "root",
+ "children": [
+ {
+ "type": "paragraph",
+ "children": [
+ {
+ "type": "imageReference",
+ "identifier": "1",
+ "referenceType": "full",
+ "alt": null,
+ "position": {
+ "start": {
+ "line": 1,
+ "column": 1
+ },
+ "end": {
+ "line": 1,
+ "column": 7
+ },
+ "indent": []
+ }
+ }
+ ],
+ "position": {
+ "start": {
+ "line": 1,
+ "column": 1
+ },
+ "end": {
+ "line": 1,
+ "column": 7
+ },
+ "indent": []
+ }
+ },
+ {
+ "type": "definition",
+ "identifier": "1",
+ "title": null,
+ "link": "/xyz.png",
+ "position": {
+ "start": {
+ "line": 3,
+ "column": 1
+ },
+ "end": {
+ "line": 3,
+ "column": 14
+ },
+ "indent": []
+ }
+ }
+ ],
+ "position": {
+ "start": {
+ "line": 1,
+ "column": 1
+ },
+ "end": {
+ "line": 4,
+ "column": 1
+ }
+ }
+}