Symbol Swapping with None doesn't work
**Which version of the plugin are you using?**
[x] Data Populator for Sketch
[ ] Data Populator for Adobe XD

**Describe the bug**
Symbol swapping doesn't work unless you pass in a symbol. In [v3.0.3](https://www.datapopulator.com/sketch/changelog.html#v3-0-3), it's suggested that you can leave out the symbol name and, if the expression returns true, the symbol will be swapped out with "none" (e.g. `#swapSymbol[true]`}.

**To Reproduce**
Steps to reproduce the behavior:
1. Add a symbol instance and rename it to `#swapSymbol[true]`
2. Select the symbol and go to **Plugins > Data Populator > Populate With Preset**
3. Select **demo/contacts-conditional** as the preset and click **Populate**
4. The symbol does not hide

**Expected behavior**
The symbol should hide when the conditional returns `true` and when no symbol name is passed to `#swapSymbol`.

**Screenshots**
<img width="290" alt="Screen Shot 2019-09-20 at 5 29 57 PM" src="https://user-images.githubusercontent.com/784440/65362245-4d684080-dbcc-11e9-9285-07342a685334.png">

**Environment (please complete the following information):**
 - macOS Version: macOS High Sierra 10.14.6
 - Sketch Version: 58
 - Plugin Version: 3.0.3

**Additional context**
I'm not getting any errors about the expression being invalid.
