Check for empty `NFTokenMinter` in `AccountSet_get_errors`

[ckniffen]

An empty string value for NFTokenMinter is not the correct way to clear out the field. This should be caught in _get_errors which is called by validate on transaction models.

See:

• https://xrpl.org/nftoken-authorized-minting.html#unassigning-an-authorized-minter
• Setting NFTokenMinter to '' causes it to be set to 'rrrrrrrrrrrrrrrrrrrrrhoLvTp' (Version: 1.9.4) rippled#4405