In Picard 2.6.3 performers without role get saved to a TXXX:performer custom field, which seems to be an undocumented and maybe unintended behavior, see this post for further details.

In terms of ID3v2.4 compliance, performers without role should get saved to TMCL (IPLS for ID3v2.3 respectively) frames as such. If empty roles are not possible, something generic like "without role" should be used.