Replace empty maps with an empty response

Instead of sending an empty CBOR map as a response, we now always send
an empty response.  Previously, we had this as a special case for the
ClientPin response, but we need it for LargeBlobs too.  So a generic
implementation makes more sense and is easier to maintain.

src/ctap2.rs

@@ -153,22 +153,21 @@ impl Response {
         let outcome = match self {
             GetInfo(response) => cbor_serialize(response, data),
             MakeCredential(response) => cbor_serialize(response, data),
-            ClientPin(response) => {
-                if response.is_empty() {
-                    Ok([].as_slice())
-                } else {
-                    cbor_serialize(response, data)
-                }
-            }
+            ClientPin(response) => cbor_serialize(response, data),
             GetAssertion(response) | GetNextAssertion(response) => cbor_serialize(response, data),
             CredentialManagement(response) => cbor_serialize(response, data),
             LargeBlobs(response) => cbor_serialize(response, data),
             Reset | Selection | Vendor => Ok([].as_slice()),
         };
         if let Ok(slice) = outcome {
             *status = 0;
-            let l = slice.len();
-            buffer.resize_default(l + 1).ok();
+            // Instead of an empty CBOR map (0xA0), we return an empty response
+            if slice == [0xA0] {
+                buffer.resize_default(1).ok();
+            } else {
+                let l = slice.len();
+                buffer.resize_default(l + 1).ok();
+            }
         } else {
             *status = Error::Other as u8;
             buffer.resize_default(1).ok();

src/ctap2/client_pin.rs

@@ -73,12 +73,6 @@ pub struct Response {
     pub retries: Option<u8>,
 }
 
-impl Response {
-    pub fn is_empty(&self) -> bool {
-        self.key_agreement.is_none() && self.pin_token.is_none() && self.retries.is_none()
-    }
-}
-
 #[cfg(test)]
 mod tests {