MD014 should not warn for empty blocks.

lib/rules.js

@@ -342,10 +342,11 @@ module.exports = [
     "func": function MD014(params, errors) {
       [ "code_block", "fence" ].forEach(function forType(type) {
         filterTokens(params, type, function forToken(token) {
+          var allBlank = true;
           if (token.content && token.content.split(shared.newLineRe)
             .every(function forLine(line) {
-              return !line || /^\$\s/.test(line);
+              return !line || (allBlank = false) || /^\$\s/.test(line);
-            })) {
+            }) && !allBlank) {
             errors.push(token.lineNumber);
           }
         });

test/code_block_dollar_fence-empty.json

@@ -0,0 +1,6 @@
+{
+  "default": true,
+  "MD009": {
+    "br_spaces": 2
+  }
+}

test/code_block_dollar_fence-empty.md

@@ -0,0 +1,14 @@
+# Header
+
+```js
+
+```
+
+```css
+
+
+```
+
+```spaces
+  
+```