[PM-29556] Fix: changing organization plan nulls out public and private keys (#6738)

Main fix: only assign new key value where old keys are not set
and new keys have been provided.

Refactors:
- use consistent DTO model for keypairs
- delete duplicate property assignment for new orgs

src/Api/AdminConsole/Models/Request/Organizations/OrganizationNoPaymentCreateRequest.cs

@@ -110,10 +110,9 @@ public class OrganizationNoPaymentCreateRequest
                 BillingAddressCountry = BillingAddressCountry,
             },
             InitiationPath = InitiationPath,
+            Keys = Keys?.ToPublicKeyEncryptionKeyPairData()
         };
 
-        Keys?.ToOrganizationSignup(orgSignup);
-
         return orgSignup;
     }
 }