Update raven.js to work around IE8 shortcomings

Internet Explorer 8 chokes on an undefined or null reference passed into the joinRegExp function:
```
SCRIPT5007: Unable to get property 'source' of undefined or null reference 
raven.js, line 1762 character 9
```

Line 1762, character 9 is as follows: 
```javascript
sources[i] = isString(patterns[i]) ?
```

Checking for the presence of an undefined or null value correctly handles this quirk while still seemingly performing correctly when present.