prevent errors when using anchors have no hrefs

the goal here is to prevent this error

```
Cannot call method 'match' of null TypeError: Cannot call method 'match' of null
    at Object.core.resourceLoader.resolve (../node_modules/jsdom/lib/jsdom/level2/html.js:84:14)
```

this error is triggered when processing html files with anchors that have no hrefs in them. 

this pull request contains tests that would fail without the code change that is also in this pull request.