Fix crash when sync() is called with no annotations

This function has no tests and I don't know what it does but sometimes
`undefined` is passed to it as the `annotations` arg and it crashes, so
fix that.

src/annotator/annotation-sync.js

@@ -61,10 +61,8 @@ AnnotationSync.prototype.sync = function(annotations) {
     return function(err, annotations) {
       var i;
       var parsedAnnotations = [];
+      annotations = annotations || [];
 
-      if (annotations === null) {
-        annotations = [];
-      }
       for (i = 0; i < annotations.length; i++) {
         parsedAnnotations.push(_this._parse(annotations[i]));
       }