gRPC: allow nil ValidationRecords in FinalizeAuthz2 (letsencrypt#4270)

The SA `FinalizeAuthorization2` RPC is used with
`FinalizeAuthorizationRequest` objects that may have a nil
`ValidationRecords` field (notably for DNS-01 challenges that
failed). The RPC wrapper should not reject such messages as incomplete.

We don't typically unit test gRPC wrappers, and adding
an integration test for this will likely conflict with
letsencrypt#4241 so I tested this
fix manually using Certbot and a local Boulder instance configured
with the authz2 feature flag.

Before applying the fix, failing a DNS-01 challenge left the
authorization stuck in pending state and Certbot would poll until
it gave up. On the server-side a 500 error matching what we observed
in staging is logged: > boulder-ra [AUDIT] Could not record updated
validation: err=[rpc error: code = Unknown desc = Incomplete gRPC
request message] regID=[xxx] authzID=[xxxx]

After applying the fix failing a DNS-01 challenge caused the associated
authorization to be marked invalid immediately. No 500 errors are
logged.

Resolves letsencrypt#4269

grpc/sa-wrappers.go

@@ -1209,7 +1209,7 @@ func (sas StorageAuthorityServerWrapper) GetAuthorizations2(ctx context.Context,
 }
 
 func (sas StorageAuthorityServerWrapper) FinalizeAuthorization2(ctx context.Context, req *sapb.FinalizeAuthorizationRequest) (*corepb.Empty, error) {
-	if req == nil || req.ValidationRecords == nil || req.Status == nil || req.Attempted == nil || req.Expires == nil || req.Id == nil {
+	if req == nil || req.Status == nil || req.Attempted == nil || req.Expires == nil || req.Id == nil {
 		return nil, errIncompleteRequest
 	}