Shinkenplugins - Create perfdata with none values

@matthieucan 

You can now create PerfData with None values without "None" appearing in the output.

Instead of doing
```
        p = PerfData(
            'blabla', 565456,
            warn=args.get('warning') or '',
            crit=args.get('critical') or '',
            min=0,
        )
```

I can now do

```
        p = PerfData(
            'blabla', 565456,
            warn=args.get('warning'),
            crit=args.get('critical'),
            min=0,
        )
```

Without having to worry about None values.