Do not throw an error when calling `contrastingColor` without a color.

I added a not supported `bsStyle` for an `Alert`. This results in a call of `contrastingColor` with `undefined` for the `color` parameter, which results in the error page the stack trace:

```
Error: unknown format:
at new Color (webpack://__%5Bname%5D/./node_modules/chroma-js/chroma.js?:179:19)
at chroma (webpack://__%5Bname%5D/./node_modules/chroma-js/chroma.js?:197:13)
at Object.contrastingColor (http://localhost:8080/app.js:92459:66)
```

We should e.g. just throw a warning in this case. It is better to see the page with an element that does not has the correct color than seeing the error page.

* Graylog Version: 3.3.5