Don't reset the MultiKernelManager's default kernel name if the Kerne…

…lSpecManager's default kernel name is set to be empty

jupyter_server/services/kernels/kernelmanager.py

@@ -723,7 +723,10 @@ def _default_default_kernel_name(self):
 
     @observe("default_kernel_name")
     def _observe_default_kernel_name(self, change):
-        if hasattr(self.kernel_spec_manager, "default_kernel_name"):
+        if (
+            hasattr(self.kernel_spec_manager, "default_kernel_name")
+            and self.kernel_spec_manager.default_kernel_name
+        ):
             # If the kernel spec manager defines a default kernel name, treat that
             # one as authoritative.
             kernel_name = change.new
@@ -740,6 +743,8 @@ def _observe_default_kernel_name(self, change):
     def _on_kernel_spec_manager_default_kernel_name_changed(self, change):
         # Sync the kernel-spec-manager's trait to the multi-kernel-manager's trait.
         kernel_name = change.new
+        if kernel_name is None:
+            return
         self.log.debug(f"KernelSpecManager default kernel name changed: {kernel_name}")
         self.default_kernel_name = kernel_name