Add suspected missing if/else case to classes.py

[LachJones]

So I think the __convert() function in classes.py was missing the case where the product parameter is set to "MatchString". I've inferred that this is an intended option for this parameter due to the behaviour of https://github.com/vehemont/nvdlib/blob/main/nvdlib/cpe.py#L339, where this value is passed through.

If this is not an intended value / not a type which should be returned from this function please let me know and I'm happy to take a look at cpe.py and submit a different PR with a more appropriate value being passed to __convert().

This PR depends on #49 being merged first in order to build on the type hinting implemented in that - apologies for this.