Don't save null permissions/tags/text to drafts store

This is a translation of commit 363134d28cb7ff8d2313278e1b4b6c09b5bf9bed
from CoffeeScript to JavaScript. Original commit message from
363134d28cb7ff8d2313278e1b4b6c09b5bf9bed:

When saving a draft to the drafts store, don't save changes for fields
which aren't actually set. This prevents us from accidentally later
restoring `null` to one of these fields.

Fixes an issue where creating an annotation when logged-out would result
in a null permissions field.

h/static/scripts/directive/annotation.js

@@ -328,11 +328,17 @@ function AnnotationController(
     // Drafts only preserve the text, tags and permissions of the annotation
     // (i.e. only the bits that the user can edit), changes to other
     // properties are not preserved.
-    drafts.update(model, {
-      text: draft.text,
-      tags: draft.tags,
-      permissions: draft.permissions
-    });
+    var changes = {};
+    if (draft.text) {
+      changes.text = draft.text;
+    }
+    if (draft.tags) {
+      changes.tags = draft.tags;
+    }
+    if (draft.permissions) {
+      changes.permissions = draft.permissions;
+    }
+    drafts.update(model, changes);
   };
 
   /**

h/static/scripts/directive/test/annotation-test.js

@@ -725,13 +725,27 @@ describe('annotation', function() {
     it('creates a draft when editing an annotation', function() {
       createDirective();
       controller.edit();
-      assert.calledWith(fakeDrafts.update, annotation, {
-        text: annotation.text,
-        tags: annotation.tags,
-        permissions: annotation.permissions
-      });
+      assert.calledWith(fakeDrafts.update, annotation);
     });
 
+    it(
+      'creates a draft with only editable fields which are non-null',
+      function() {
+        // When a draft is saved, we shouldn't save any fields to the draft
+        // "changes" object that aren't actually set on the annotation. In this
+        // case, both permissions and tags are null so shouldn't be saved in
+        // the draft.
+        createDirective();
+        annotation.permissions = null;
+        annotation.text = 'Hello!';
+        annotation.tags = null;
+
+        controller.edit();
+
+        assert.calledWith(fakeDrafts.update, annotation, {text: 'Hello!'});
+      }
+    );
+
     it('starts editing immediately if there is a draft', function() {
       fakeDrafts.get.returns({
         tags: [