fix(controlplane): guard against empty billing owner emails

[mekilis]

Summary

• stop Convoy billing sync from sending blank owner emails by short-circuiting when the resolved owner email is empty
• return a clear 422 response when billing bootstrap cannot resolve organisation owner email
• add a focused service test that ensures billing create is not called when owner email is empty

Test plan

• go test ./services -run TestRunBillingOrganisationSync
• go test ./api/handlers -run \"TestGetInternalOrganisationID_BillingDisabled_DoesNotCallBilling|TestIsBillingOrgNotFound\"