Fix region nonfungible implementation (#5067)

The problem with the current implementation is that minting will cause
the region coremask to be set to `Coremask::complete` regardless of the
actual coremask.

This PR fixes that.

More details about the nonfungible implementation can be found here:
https://github.com/paritytech/polkadot-sdk/pull/3455

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Co-authored-by: Dónal Murray <donalm@seadanda.dev>
Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>