[IZPACK-1131] - mustExist attribute for file fields

This is to address: https://jira.codehaus.org/browse/IZPACK-1131

Add a mustExist attribute for file fields, just like in dir fields.
If mustExist is not specified default is true.
```
<field type="file" align="left" variable="non.file" summaryKey="non.file.description">
    <description id="non.file.description"/>
    <spec size="34" mustExist="false"/>
</field>
```

Also ensure not to print "null" during console installation when no label exists.