Fixed JoinSqlBuilder for SqlLite

With SqlLite provider the FieldDefinition.Alias doesn't default to FieldDefinition.Name when the field doesn't have an Alias attribute like in SqlServer or Firebird.

Added a check for null or empty to FieldDefinition.Alias before use.
```
var fieldName = String.IsNullOrEmpty(fieldDef.Alias) ? fieldDef.Name : fieldDef.Alias;
```

Also fixed the UserEx class in JoinTest.cs by adding a missing BelongTo attribute to CreatedDate property.
```
[BelongTo(typeof(User))]
public DateTime CreatedDate { get; set; }
```