Fix Null Prevalue Values on Export of DataTypes

Exporting a DataType that has a Numeric pre-value that has no value
causes an Exception - 

```
System.ArgumentNullException: Value cannot be null.
Parameter name: value
   at System.Xml.Linq.XAttribute..ctor(XName name, Object value)
   at Umbraco.Core.Services.PackagingService.Export(IDataTypeDefinition dataTypeDefinition, Boolean raiseEvents)
   at jumps.umbraco.usync.SyncDataType.SaveToDisk(IDataTypeDefinition item)
```

added null checking to pre value.value