client-api: Don't require the failures field in the signatures upload response

The field is not marked as required in the spec[1] and at least one homeserver, namely Dendrite, omits the field if it's empty.

[1]: https://spec.matrix.org/unstable/client-server-api/#post_matrixclientv3keyssignaturesupload

crates/ruma-client-api/CHANGELOG.md

@@ -1,5 +1,10 @@
 # [unreleased]
 
+Bug fixes:
+
+- Don't require the `failures` field in the
+  `ruma_client_api::keys::upload_signatures::Response` type.
+
 Breaking changes:
 
 - The conversion from `PushRule` to `ConditionalPushRule` is infallible since

crates/ruma-client-api/src/keys/upload_signatures.rs

@@ -45,6 +45,7 @@ pub mod v3 {
     #[derive(Default)]
     pub struct Response {
         /// Signature processing failures.
+        #[serde(default, skip_serializing_if = "BTreeMap::is_empty")]
         pub failures: BTreeMap<OwnedUserId, BTreeMap<String, Failure>>,
     }
 
@@ -117,14 +118,17 @@ pub mod v3 {
         _Custom(PrivOwnedStr),
     }
 
-    #[cfg(all(test, feature = "client", feature = "compat-upload-signatures"))]
+    #[cfg(all(test, feature = "client"))]
     mod tests {
-        use ruma_common::user_id;
-
-        use super::{FailureErrorCode, ResponseBody};
+        use super::ResponseBody;
 
+        #[cfg(feature = "compat-upload-signatures")]
         #[test]
         fn deserialize_synapse_response() {
+            use ruma_common::user_id;
+
+            use super::FailureErrorCode;
+
             const JSON: &str = r#"{
                 "failures": {
                     "@richvdh:sw1v.org": {
@@ -142,6 +146,14 @@ pub mod v3 {
             assert_eq!(failure.errcode, FailureErrorCode::InvalidSignature);
             assert_eq!(failure.error, "400: Invalid signature");
         }
+
+        #[test]
+        fn deserialize_empty_response() {
+            const JSON: &str = r#"{}"#;
+
+            let _parsed: ResponseBody = serde_json::from_str(JSON)
+                .expect("We should be able to deserialize an empty keys/signatures/upload");
+        }
     }
 }