fix: filter empty messages in toModelMessage (#4811)

packages/opencode/src/session/message-v2.ts

@@ -666,7 +666,7 @@ export namespace MessageV2 {
       }
     }
 
-    return convertToModelMessages(result)
+    return convertToModelMessages(result.filter((msg) => msg.parts.length > 0))
   }
 
   export const stream = fn(Identifier.schema("session"), async function* (sessionID) {