Fix bug in latest filter package

"If the value attribute is not specified, the content will be passed as a value instead." (http://www.w3schools.com/tags/att_option_value.asp - the most reliable source ever).

Michael stated in #579 that the values from the database where

```
["rel_type"] => string(0) ""
["milestone_id"] => string(12) "No Milestone"
```

This fixes that by specifying a default value of 0 for both fields (which isn't a possible relation for the 2 fields). If this value is met then in the model they are set to null (note I don't know whether these checks for 0 are better done in model or table - I was strongly tempted to put them into App\Tracker\Table\IssuesTable::check() function tbh)

Note I have absolutely no virtual box for this thing set up so you MUST test this before merging