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
+    }
+  }
+}