fix(rpc): serialize traces always as vec (#3770)

crates/revm/revm-inspectors/src/tracing/builder/parity.rs

@@ -138,7 +138,12 @@ impl ParityTraceBuilder {
 
         let (trace, vm_trace, state_diff) = self.into_trace_type_traces(trace_types);
 
-        TraceResults { output: output.into(), trace, vm_trace, state_diff }
+        TraceResults {
+            output: output.into(),
+            trace: trace.unwrap_or_default(),
+            vm_trace,
+            state_diff,
+        }
     }
 
     /// Consumes the inspector and returns the trace results according to the configured trace

crates/rpc/rpc-types/src/eth/trace/parity.rs

@@ -30,12 +30,13 @@ pub enum TraceType {
 pub struct TraceResults {
     /// Output of the trace
     pub output: Bytes,
-    /// Enabled if [TraceType::Trace] is provided
-    pub trace: Option<Vec<TransactionTrace>>,
-    /// Enabled if [TraceType::VmTrace] is provided
-    pub vm_trace: Option<VmTrace>,
     /// Enabled if [TraceType::StateDiff] is provided
     pub state_diff: Option<StateDiff>,
+    /// Enabled if [TraceType::Trace] is provided, otherwise an empty vec
+    #[serde(default)]
+    pub trace: Vec<TransactionTrace>,
+    /// Enabled if [TraceType::VmTrace] is provided
+    pub vm_trace: Option<VmTrace>,
 }
 
 /// A `FullTrace` with an additional transaction hash