Print imageReference with no alt in Markdown (#3643)

src/language-markdown/printer-markdown.js

@@ -284,7 +284,7 @@ function genericPrint(path, options, print) {
     case "imageReference":
       switch (node.referenceType) {
         case "full":
-          return concat(["![", node.alt, "][", node.identifier, "]"]);
+          return concat(["![", node.alt || "", "][", node.identifier, "]"]);
         default:
           return concat([
             "![",

tests/markdown_imageReference/__snapshots__/jsfmt.spec.js.snap

@@ -14,6 +14,13 @@ exports[`full.md 1`] = `
 
 `;
 
+exports[`full-no-alt.md 1`] = `
+![][world]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+![][world]
+
+`;
+
 exports[`shortcut.md 1`] = `
 ![hello]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

tests/markdown_imageReference/full-no-alt.md

@@ -0,0 +1 @@
+![][world]