if you don't fill the field with the version of the file during publishing update the resource will not be updated by new file.
Old file will remain in resource, although user gets notification about successfully resource update and there are no notification about empty version field (!) .
Can do you change it in such way:
1) If user don't fill the field, than engine should not permit update and warning notification will be prompted "You should fill version field".
2) If user enter old version, than old file remains in resources (like an old behaviour).