Fix initial link setting

Without this fix, on extension initialization:

`localStorage['open-on-click'] === undefined`
`localStorage.getItem('open-on-click') === null`

JavaScript, in its infinite wisdom, assigns a _string_ "null" here:
`link.defaultValue = link.value = bg.getLink();`

Therefore, upon opening the options page the link field is filled with "null", and if options are saved and the default link is prevented from being used.