`undefined` css code blocks in /source

If a brew doesn’t have any css added via the editor panel, the brew when viewed via /source will have a css code-block prepended that looks like this:

```css
undefined
```

That’s a bug, easy to fix.

Also appears in the /download resource.

[Gazook89]

So #1850 does this:

1. If style tab isn’t touched from “new”, the example styling doesn’t appear, and no code fence block.
2. If it’s changed, the change appears
3. If it’s changed, but entirely deleted, it shows the code fence block but is empty:

```css

```

[ericscheid]

@calculuschild testing just now and I concur. It must have been an old brew I was looking at. What we can do is make sure we don’t consume the string “undefined” from an old brew and stick that into the style editor.

Ah, found these two in my downloads folder:

https://homebrewery.naturalcrit.com/share/HJ-xEkR-77 https://homebrewery.naturalcrit.com/share/1Xy5P5f_d4aYQ8mSSTATBQtKuOLqI8Qs7N7hF0h2ml8Gf

Since this is a minor glitch of bad data that won’t cause any problems, we can safely ignore.