Add type to supported_features

ipykernel/kernelbase.py

@@ -880,23 +880,23 @@ async def connect_request(self, socket, ident, parent):
 
     @property
     def kernel_info(self):
+        from .debugger import _is_debugpy_available
+
         info = {
             "protocol_version": kernel_protocol_version,
             "implementation": self.implementation,
             "implementation_version": self.implementation_version,
             "language_info": self.language_info,
             "banner": self.banner,
             "help_links": self.help_links,
-            "supported_features": [],
         }
+        supported_features: list[str] = []
         if self._supports_kernel_subshells:
-            info["supported_features"] = ["kernel subshells"]
-
-        from .debugger import _is_debugpy_available
-
+            supported_features.append("kernel subshells")
         if _is_debugpy_available:
-            info["supported_features"].append("debugger")
-
+            supported_features.append("debugger")
+        if len(supported_features) > 0:
+            info["supported_features"] = supported_features
         return info
 
     async def kernel_info_request(self, socket, ident, parent):